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151

Unit 2 is it all in the Genes?

Chapter 5 Molecular Genetics

Chapter 6 inheriting Genes

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Molecular Genetics

essentials

Melanin Pigment in skin provides a dark color. It darkens skin as sunlight penetrates its layers

DNA Replication makes more of itself passing itself onto new generations

Mutation on a gene

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DNA provides the instructions for life, including skin color

Tanzanian Albinism ©

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154 Unit 2: Is it all in the Genes?

the Case of Out-of-place Color “It was a terrible sight,” recalled the emergency room attending physician, Dr. Franc. Fourteen-year-old Joyce Carl had been rushed into the ER last week with bruises and cuts on her left arm and shoulder, after a man tried to abduct her while she was walking home from school. Joyce resisted, and the aggressor tried to cut off her arm. A group of onlookers helped her to escape.

Even uninjured, Joyce would be a strange spectacle here in Dar es Salaam, Tanzania. An albino, she had white skin, blonde hair, and pale blue eyes, making her stand out dramatically among her black peers. Albinism is a noncontagious disease that is rare in most parts of the world but fairly common in Tanzania, affecting one in 2,000 people. Genetically inherited from both mother and father, it results from a lack of pigmenta- tion. Eyes, skin, and hair are without color, and individuals with the disorder are highly susceptible to skin cancers and burns.

Dr. Franc also knew that in Tanzania, as well as other parts of sub-Saharan Africa, albinos are believed to have magical powers. It is not a compliment to their difference. Witch doctors sell albinos’ hair, skin, bones, and internal organs on the open market as ingredients for potions that are supposed to make people rich. With an arm going for $2,000, about 20 Tanzanian albinos are killed each year for their body parts. This is a great deal of money in developing economies, equivalent to over USD $200,000. Joyce was almost a victim, and an estimated 170,000 albinos live in fear.

Dr. Franc told Joyce she would leave tomorrow morning, given that her condition was improving. She was a friendly and well-adjusted young lady and he wished her well. But as he was leaving the room she asked him, “Why is this happening to me?” Dr. Franc knew that human genetics is a powerful force in society as well as in our bodies.

early ideas about Genetics At times, a very obvious family trait is handed down from generation to generation. Consider the distinctive facial features of the Hapsburgs, the royal family of the Austrian Empire in Europe, which dominated the political scene there from 1282 to1918. Many of its members had a protruding lower lip that became associated with the wealthy upper class of old Austria (Figure 5.1).

While it was easy to observe certain physical characteristics, like the protrud- ing lower lip, which have passed from one generation to the next, understanding how

Albinism

Is a noncontagious disease that is genetically inherited and results from a lack of pigmentation.

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From reading this chapter, students will be able to:

• Examine how genetics affects society and our everyday lives. • Explain the scientific development of big ideas in genetics and life’s origins. • Describe and draw DNA structure and compare it with RNA. • Explain the process of DNA replication. • Use base sequences to view DNA as the universal language of genetics, and connect DNA to protein

production via the processes of transcription and translation. • Analyze errors in gene regulation, connecting to such diseases as albinism and cancer.

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CheCk Up seCtiOn

There are examples of past and present discrimination in the U.S. society based on genetic differences. Choose a particular case to research. Explain parallels you see between it and the discrimination seen for Tanzanian albinos.

characteristics were passed on came only recently. Life arises by using information passed down from parents. The information chemical was not discovered until much later in our history. This chapter will explore the structure and function of this inherited information.

Underlying the question of inherited characteristics is a more basic one: how did life arise? Some thought we spontaneously developed without a need for parents: that life simply arose on its own, nurtured within a womb. The theory that life could arise from nonlife is termed spontaneous generation. This question has been pondered and answers posed at least since the Greek philosopher Aristotle. The first recorded scientific consid- eration for the question of how life began came from Aristotle. He believed that a male’s semen was an imperfect mixture of materials that, when united with “female semen,” would combine to make a more perfect human offspring. Nothing more than this was known about how our species was formed.

Figure 5.1 Kaiserin (Empress) Maria Theresia of the Austrian Empire—The Haps- burg Royal Family. The protruding lips of the Hapsburg family members clearly identify them as related to one another.

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Ideas on spontaneous generation went generally unchallenged for over 3,000 years. Then, in 1677, Anton van Leeuwenhoek, the Dutch lens maker (discussed in an chapter 3), observed living material under his newly developed microscope. He described the “little animals or animalcules” in the many water samples he took from lakes. These are what we now know to be microorganisms or microbes. He saw bacteria and fungi on cheese, bacte- ria in his saliva, and sperm from his own samples (his own sperm sample he did not pub- licly disclose due to religious rules at the time). Van Leeuwenhoek described the objects he saw as “little eels or worms, lying all huddled up together and wriggling. …This was for me, among all the marvels that I have discovered in nature, the most marvelous of all.” But it was Robert Hooke who first coined the term “cell” to describe structures composing the plants he observed, which looked to him like a monk’s cell or quarters in a monastery (as discussed in earlier chapters).

The sperm van Leeuwenhoek and several contemporaries observed, were described as little humans encased in a special cell with a tail. Thus, they reasoned, any resem- blance of a child to its mother was due to her internal chemicals influencing the fetus’ development - that a fully formed human came from fathers. Figure 5.2 depicts the image van Leeuwenhoek and his contemporaries claimed to have seen under the microscope. We now know that these images were sperm cells with a nucleus and not a fully formed organism. The sperm’s composition is quite a bit different from what van Leeuwenhoek supposed. Ideas about the start of life changed over the centuries and remain a source of public debate, as discussed in Bioethics Box 5.1.

Further experiments on plants and animals yielded a change in thinking. From work on the pollination of flowers and trees, it became clear that both male and female parents con- tributed to the next generation of plants. Knowledge about animal reproduction advanced to show that it took a fusion of egg and sperm to create a new organism. This understanding raised the question: What exactly is being inherited by the offspring? Spontaneous genera- tion was disproved in an experiment by Louis Pasteur in the mid-1800s. In his experiment, he constructed special flasks with elongated necks to keep out microbes. He showed that life would appear only from other life. However, a clear mechanism explaining how organ- isms inherited information from parents had not yet been suggested.

Gregor Mendel, an Austrian monk working in his garden in the late 1800s, studied pea plants and their changing characteristics through successive generations. He noted that certain patterns of inheritance emerged, and that predictions about offspring could

Animalcules

The dated term for a microscopic animal, we now know of as microorganisms.

Figure 5.2 Homunculus, (little man) a future human being, preformed in a human sperm.

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be made from observations about the mating parents. His studies are considered to mark the birth of genetics and give Gregor Mendel the title of “founder of genetics” as a modern discipline of study. Mendel’s conclusions will be discussed in detail in the next chapter. The laws of inheritance described by Mendel can explain Joyce Carl’s albinism.

At about the same time that Mendel was making observations about patterns of inheritance, other scientists had observed structures within cells that moved apart when a cell divided. These structures were called chromosomes, which are compact bodies that are inherited from one cell generation to the next (as discussed in chapter 3). Exper- imentation showed that chromosomes were made up of two substances: proteins and deoxyribonucleic acid (DNA).

The discovery of DNA was made by Friedrich Miescher, a German chemist who in 1869 extracted a substance from the nuclei of cells he was working with. This substance was white, slightly acidic, and contained phosphorous; Miescher called it an organic acid. However, little work was done on DNA for decades, because it was not known to be the agent of heredity. Only in the mid-20th century did biochemists find out that DNA was hereditary material; this discovery unleashed a flurry of research to determine its molecular structure.

To do this, early in 1928, Frederick Griffith, a British microbiologist, conducted experiments on mice to determine which material is inherited: proteins or DNA? He studied the effects of a pathogenic (disease-causing) strain of Streptococcus pneumonia bacteria. He unexpectedly noticed that when he injected mice with a set of different strains of the same bacteria, sometimes they would not get sick. There were two forms of S. pneumonia: the R- strain, which lacks a polysaccharide coat making it appear rough and an S-strain, which had a coat, making it appear smooth in shape. He deter- mined that those bacteria having a surrounding polysaccharide coat (the S-strain) were pathogenic, or disease causing. The coat must have conferred some sort of protection from the immune system of the mouse that allowed coated bacteria to cause the disease. When Griffith heated the coated S-strains to kill them and injected the dead bacteria into the mouse, the mouse lived. Bacteria were not able to hurt the mouse because they were dead. When Griffiths mixed heat-killed virulent S-strain bacteria and live nonpathogenic R-strain bacteria, however, the mouse died. How could a dead, nonpathogenic bacteria kill the mouse? Griffith concluded that a chemical possessed by the heat-killed virulent bacteria must be a transforming agent. It changed the living R-strain bacteria from one type to another, into a S-strain type (see Figure 5.3). In 1944, Oswald Avery, a professor at the Rockefeller University, isolated and analyzed this chemical, determining it to be deoxyribonucleic acid. The story of Joyce Carl is based on this mystery material and how it leads to skin color. We will explore the role of DNA in determining our unique characteristics as this chapter unfolds.

Thus, it became clear, through a series of experiments in the early part of the 20th century, that 1) DNA was inherited; and 2) DNA was the chemical that directed new cell production as well as all cellular activities. However, it was unclear what this new molecule looked like and how it actually worked.

Many scientists contributed their ideas and expertise to discovering the shape of DNA. U.S. chemist Linus Pauling showed that protein chains of amino acids were helical in shape, like a slinky, and were held together by hydrogen bonds between successive turns. He suggested that the DNA molecule could resemble such a structure, and he was right. X-ray diffraction showed that there were turns in the DNA molecule and that certain chemicals, known as nitrogenous bases, occurred in regular, repeating patterns.

• Nitrogenous bases, as described in Chapter 2, are an important component of nucleotides, the building blocks of DNA.

Chromosomes

Structures within cells that move apart when a cell divides.

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Genetic Transformation A

Figure 5.3 Griffiths Experiment. Bacterial transformation, as show in the figure, was the key to Griffith’s experiement. Bacteria ingest genetic material from the environ- ment and begin to exhibit those chracteristics of that genetic material. As he studied vaccines for pneumonia, Griffith discovered that bacteria could mutate quickly. One dead bacterial cell (heat-killed S-cell) could be consumed by another bacteria (R-cell), transforming it into another type of S-cell. It is a bit like a horror story, eating another creature and becoming just like it.

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Bases adenine and thymine occurred in the same proportions, and guanine and cytosine also occurred together in the same proportions. This finding led to further discovery about DNA structure. The repeating patterns of the bases indicated that they must be paired together in a certain way.

When English scientist Francis Crick and American James Watson were working in Cambridge University in 1953, they used information from several sources to develop a new model for DNA. The 23-year-old James Watson, a newly minted PhD traveled to London to visit the lab of Maurice Wilkins at King’s College. There he discovered an X-ray image of DNA taken by Rosalind Franklin, Wilkins’s colleague (Figure 5.4). Wat- son studied the image to deduce the shape of DNA to be of a certain size and shape, and from that developed a clue for the model. Franklin’s work indeed gave Watson the idea for his model, but she did not receive credit in the publication describing the arrange- ment of DNA, as described in the Bioethics Box 5.2.

Watson and Crick put all the research from the varied sources together, figuring out just how DNA is inherited from generation to generation. This work represented the birth of molecular genetics, a new field that united biology, chemistry, and genetics, to study inheritance at the chemical level. Inheritance could now be explored at its most elemental level.

Watson and Crick’s model is used to explain many aspects of chemical inheritance, such as: the way in which DNA reproduced itself; the way it is transformed into protein for a cell’s use; and how DNA directs the many activities within the cell. Watson and Crick’s discovery is more than a simple description of a chemical; it is the basis for explaining how information is passed on from generation to generation and within cells. Their model will be used throughout this chapter to describe that information flow.

Molecular genetics

A new field that united biology, chemistry and genetics, to study inheritance at the chemical level.

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Figure 5.4 DNA uncoiled. DNA contains weak bonds holding the two strand of DNA together. These break easily allowing DNA to unzip and expose the base pairs. The sugar-phosphate backbone of the DNA molecule provides support to the structure. Note its base pairs, which, when exposed, gives DNA its unique informational message. From Biological Perspectives, 3rd ed by BSCS.

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Dna as an inherited substance the structure of Dna Watson and Crick showed that DNA resembles a twisted ladder, with sugars and phosphates on the vertical parts of the ladder and bases, making up the rungs of the ladder. The sugars and phosphates comprise the back- bone of the DNA molecule. This type of structure is known as a “double helix.”As discussed in Chapter 2, DNA

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160 Unit 2: Is it all in the Genes?

is a nucleic acid, a macromolecule that stores information – the “code of life” – in strings of base sequences. Each of these bases constitutes a code to guide the cell’s activities. The exact structure of DNA, as sketched out by Watson and Crick, is shown in Figures 4.5 and 5.5. Sugars and phosphates hold the up-down portions of the molecule together while bases pair with each other in the horizontal levels of the DNA in the figure.

The basic functional unit of a DNA molecule is the nucleotide, which is made of three parts: a sugar backbone, either ribose, found in RNA, or deoxyribose, found in DNA; a phosphate group, which contains four oxygen atoms bound to a central phos- phate and is negatively charged and very acidic; and a nitrogen-containing molecule that is the base. Bases make up the genetic code. There are four types of bases: adenine, thymine, guanine and cytosine, commonly written as A, T, G, and C, but with U (uracil) instead of T (thymine) in RNA. Bases are held together by weak hydrogen bonds that are able to be taken apart relatively easily when they are used to direct the cell’s activities or make new DNA. (Figure 5.4 and Figure 5.7 shows all of these components.) Bases comprise a set of directions for the cell much like a set of directions used to find a par- ticular location or address.

Just as you may arrive at the wrong place if there is a mistake in the directions, a cell may have problems when its DNA has errors. In the case of cells, an error in a base in the DNA sequence is called a mutation and can lead to problems for living cells. Mutations are responsible for a number of diseases that will be treated in the next chapters. Muta- tions are sometimes caused by environmental factors. They are an example of how the environment has an influence on how DNA becomes expressed. For example, factors in the environment, such as radiation or harmful substances, increase the risk of mutations and therefore change the genetic structure (Figure 5.6). Changes in DNA’s structure also lead to changes in organisms’ characteristics. Mutations are what led to Joyce Carl’s albinism because a simple change in nucleotide sequence is the cause of her lighter skin.

Bases couple together specifically: adenine always pairs with thymine (A-T) and guanine always pairs with cytosine (G-C), for example. This special coupling is called

Nucleotide

The basic functional unit of a DNA molecule.

Ribose

The sugar backbone found in RNA.

Deoxyribose

The sugar backbone found in DNA.

DNA

A long macromolecule containing the information code that directs cellular activities in living organisms.

Adenine

A purine base that is a component of RNA and DNA.

Thymine

A pyrimidine base that is found in DNA but not RNA.

Guanine

A purine base that functions as a fundamental constituent of RNA and DNA.

Cytosine

A type of base found in DNA.

Uracil

A pyrimidine base that is one of the fundamental components of RNA.

CRiCk’s PeRsPeCTive oN DNA

Watson and Crick used their inductive abilities to gather information from many other scientists and publicize the model we use today. Their discovery captured the interest of the scientific community and they quickly became celebrities. This interest helped get needed research money into their devel- oping field of molecular biology. However, it takes a certain humble apprecia- tion for the molecule as a thing of beauty, to really understand its meaning in society and not the scientist’s fame. Consider the excerpt below from Francis Crick in 1974, for his reflections on this process:

Rather than believe that Watson and Crick made the DNA structure, I would rather stress that the structure made Watson and Crick. After all, I was almost totally unknown at the time and Watson was regarded, in most circles, as too bright to be really sound. But what I think is overlooked in such arguments is the intrinsic beauty of the DNA double helix. It is the molecule which has style, quite as much as the scientists. - Francis Crick, “The Double Helix: A Personal View,” Nature, 26 April 1974.

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Figure 5.5 DNA’s structure.

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complementarity. You can remember this particular base pairing by recalling the phrase: “AT the Grand Canyon,” where the initials AT connect adenine and thymine, and the Grand Canyon’s initials G and C stand for linked guanine and cytosine.

Just what material do we inherit from one generation to the next? We inherit genes. We have all heard someone saying, “It’s in your genes!” A gene is a discrete bit of data on the DNA molecule, usually a series of nucleotides that code for information to be used by the cell. A gene is the functional unit of heredity and the main player in information transfer within cells. It carries the codes for all of our characteristics. Humans have between 20,000 and 25,000 genes in their cells’ nuclei. A single gene has over 100,000 nucleotide pairs, and a DNA molecule contains over 200 million base pairs. A full set of human DNA is estimated to contain over 3 billion base pairs, or enough information to fit into 600,000 printed pages of 500 words each. In essence, that is equivalent to 1000 library books! How can all this information fit into a single cell when it is thousands of

Complementarity

The specific coupling of bases.

Gene

A portion DNA sequence serving as the basic unit of heredity.

Figure 5.6 Melanin mutation on the genetic code. A mutated melanin (gene #3 in this hypothetical example) gene does not code for a proper functioning melanin pig- ment molecule. From Biological Perspectives, 3rd ed by BSCS.

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BioeThiCs Box 5.1: Why WAs DR. RosAliND FRANkliN, A BRilliANT sCieNTisT, iGNoReD By heR ColleAGUes?

The truth behind how DNA was discovered starts with King’s College in London, 1953. Dr. Franklin was born in 1920 and earned a doctorate in phys- ical chemistry at the age of 26, against her father’s wishes. She worked at Kings College, refining X-ray diffraction to produce the “Photograph 51” that helped Watson and Crick develop their model of DNA.

When Dr. Franklin was a colleague of Maurice Wilkins, she was treated as a mere helper and suffered gender discrimination in a male-dominated field. Although her work was published in the same issue of the journal Nature, as the Watson and Crick paper, in April, 1953, she was not given credit for her contributions to the model.

Tragically, she died of ovarian cancer in 1958, at the age of 37, four years before Watson and Crick received the Nobel Prize for their work. It is likely that she died for the model . . . She worked hundreds of hours to perfect her photographs of crystallized DNA, exposing herself to large doses of radiation. The photo below shows scientist Rosalind Franklin’s X-ray image of DNA. Most scientists are aware of her contributions today, and Dr. Franklin is given posthumous credit in this textbook (Figure 5.7).

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Figure 5.7 a. and b. Watson and Crick Model of DNA. c. X-ray image of DNA by Franklin. Franklin helped Watson and Crick to develop their model of DNA structure in the 1950s.

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Figure 5.7 (continued)

G C

P P

Sugar Sugar

P P

Sugar Sugar

P P

Sugar Sugar

Sugar-phosphate backbone

Sugar-phosphate backbone + base = nucleotide

Nitrogenous bases: A: Adenine T: Thymine G: Guanine C: Cytosine

Hydrogen bond3' end 5' end

A DNA molecule consists of two spirally-wound sugar- phosphate chains linked through the hydrogen bonding of four nitrogenous bases. Adenine links with thymine while guanine pairs with cytosine.

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164 Unit 2: Is it all in the Genes?

Figure 5.7 (continued)

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times longer than the cell itself? The answer is that DNA is supercoiled, (Figure 5.8) which means that it is packaged together very tightly around histone proteins, like a shoelace wrapped around your fingers.

DNA is common to all living creatures: humans, bacteria, mushrooms, and oak trees all contain the same type of macromolecules. A full set of DNA in an organism is called its genome. In bacteria, as in all prokaryotes, the genome is naked and circular; it is not surrounded by a nucleus and occurs as a continuous series of nucleotide bases. Eukary- otes contain genomes packaged into discrete units as chromosomes. Each species has a unique, set number of chromosomes. Fruit flies have only 8, corn has 20, and dogs have 78. All eukaryotes contain chromosomes in pairs and sometimes organisms of different species have the same number of chormosomes.

Humans have a total of 46 chromosomes, with 23 pairs of them. In humans, chro- mosomes are inherited, one from a mother and another from a father. Before a cell can divide, chromosome pairs must double in number so that each new cell will contain a

histone

Group of basic proteins in chromatin.

Figure 5.8 DNA supercoiling genes on DNA are coiled extensively around histone proteins.

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full set. The chromosomes then move to two separate new cells during cell division. How the DNA makes itself into two full sets was finally answered by Watson and Crick’s model.

how Does eukaryotic Dna reproduce itself? Mitosis During roughly 90% of an average cell’s life cycle, it is actively conducting the many nor- mal cell functions described in Chapter 3. This period of time is known as the interphase for a cell. In the remaining 10% of the time, the cell divides via mitosis.

The cell cycle, or the life span phases a cell goes through, includes mitosis (see Fig- ure 5.9). The cell cycle involves the division of the cytoplasm and nucleus to produce two new identical daughter cells from one original cell. During interphase, a cell gets ready for mitosis by. doubling its genetic material, increasing its number of organelles and its cytoplasm size.

There are three phases of interphase: G1 phase, S Phase, and G2 phase (see Figure 5.9). During the G1or growth-1 phase, a cell grows rapidly in size, forming new organelles and proteins for future daughter cells. Centrioles, a special microtubule units used for mito- sis, is made during G1 of interphase. During the S, or synthesis, phase, chromosomes are synthesized to duplicate the genetic material. It is just before the start of the S phase that a cell “decides” to divide or not. Why at this point? Because once a cell enters the S phase, the large investment in doubling the DNA is too great to turn back; a cell must continue to divide into two daughter cells. The decision to either become nondividing or begin DNA synthesis depends on two major factors: 1) a cells’ cell-to-volume ratio. A cell needs a certain amount of cytoplasm to be able to function normally. As discussed in Chapter 3, that ratio is set to allow cells to transport materials to every region; and 2) the presence

interphase

The stage in cell development following two successive mitotic or meiotic divisions

Cell cycle

The life span phases a cell goes through.

G1 phase

A period in the cell cycle in which a cell grows rapidly in size, forming new organelles and proteins for future daughter cells.

s phase

A period in the cell cycle in which DNA is replicated.

G2 phase

A period in the cell cycle in which growth of the cell’s cytoplasm and organelles is completed and final preparations for mitosis takes place.

Figure 5.9 The cell cycle. Cells undergo a series of phases throughout their life cycle; growing and dividing, with checkpoints regulating the process. From Biological Perspectives, 3rd ed by BSCS.

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of MPF, mitosis-phase promoting factor. This chemical triggers mitosis by activating pro- teins that help in the cell-division process. Finally, during the G2 phase, growth of the cell’s cytoplasm and organelles is completed and final preparations for mitosis takes place.

Mitosis then occurs, with cells dividing into identical new cells. Mitosis is defined as the process by which the nucleus and nuclear components divide, resulting in two new identical cells. Mitosis produces new cells for healing cut skin, making new organs in an embryo, or giving rise to a newly created single-celled organism such as an Amoeba. The stages of mitosis are shown in Figure 5.10.

Cells are constantly being reproduced in the human body. As body cells wear out, they need to be replaced or repaired. Consider human bones: Did you know that all of our bones are replaced every five to ten years? Bones remodel continually according to a variety of factors: whether there is enough calcium for their development or whether forces are placed upon the bones through exercise to build more mass, to name a couple. Thus, cells divide to accomplish the life functions of an organism.

Figure 5.10 Stages of mitosis. A cell with four pairs of chromosomes is shown here, dividing. The stages of mitosis and their respective characteristics are given for each phase. From Biological Perspectives, 3rd ed by BSCS.

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Mitosis occurs in an orderly manner, with set steps for eukaryotic cells. The first phase of mitosis is prophase (see Figure 5.10 to follow the steps of mitosis). Prophase is characterized by chromatin being packaged into chromosomes in a cell’s nucleus. Chro- matin is the thin, strewn about form of DNA in the nucleus. Chromatin condenses to form chromosomes. Chromosomes, as dense bodies, can then be transported as discrete packages into new cells as the cell divides. During prophase, the nucleus disintegrates and centrioles move to opposite ends of the cell. The function of centrioles is not com- pletely understood, but they are thought to organize a network of spindle fibers. Spindle fibers are made up of microtubules, later used for pulling chromosomes to opposite ends of the cell.

The next phase is metaphase, during which chromosomes line up at the middle of the nucleus, the equator, attaching to the spindle fibers. Sister chromatids (identical strand of the duplicated chromosomes) attach to spindle fibers via a kinetochore, which is like a protein handle, securing chromosomes by way of a centromere, to the microtu- bules of the spindle, as shown in Figure 5.10. During anaphase, identical chromosomes move apart while attached to the spindle fibers. This physically divides genetic material to opposite sides of a cell, called poles. The mechanism believed to occur is by micro- tubules shortening, which pulls their attached chromosomes to the poles. At this point, genetic material at the poles is identical to that of its parents.

The last phase is called the telophase, during which time there is a reversal of the events occurring during prophase. Two new nuclei start to reform at the poles, chro- mosomes elongate, forming chromatin once again, and the cell’s cytoplasm pinches, forming an indentation along the equator of the cell. This final process, whereby the division of the cytoplasm takes place, is called cytokinesis. In animal cells, the first sign of cytokinesis is the formation of a shallow groove along the equator of the cell. This pinching of cytoplasm is actually a cleavage furrow formed by a pulling of microfila- ments. Quickly, much like purse strings, the cleavage furrow deepens to divide cyto- plasm, forming two new cells. In plant, algae, fungal, and some bacterial cells, a cell plate forms at the equator, with vesicles from the Golgi apparatus coalescing to form two new plasma membranes and later, two new cell walls.

Prophase

A stage that is characterized by chromatin being packaged into chromosomes in a cell’s nucleus.

Metaphase

A phase in which chromosomes line up at the middle of the nucleus, the equator, attaching to the spindle fibers.

Anaphase

A cell division stage in which chromosomes split into two identical groups and move toward the opposite poles of cells.

Telophase

A phase during which time there is a reversal of the events occurring during prophase.

Cytokinesis

The division of cell cytoplasm.

A WAy To ReMeMBeR The PhAses oF MiTosis

To help you remember these phases, mitosis can be analogous to the dating process. When your date arrives during the prophase, do you clean up your room or keep it sloppy? Of course, you tidy up your clothes and try to make a good impression. Much like in dating, a cell organizes its chromatin by making it into chromosomes. Then in the middle of the date, or metaphase, you have a good time; then you talk about too much biology, and Ana, in the anaphase, starts running away from you. The same thing occurs in the anaphase of cell division, in which chromosomes are running away to opposite poles of the cell. Have you ever experienced that heinous call or even text message which states: “It’s over between us . . . ” and dead silences or no responses show it is really the end? This series of events is analogous to the telophase. Ana is breaking up with you on the telephone just like the cell breaks up during telophase. One would hope that a breakup would be via telephone and not a mere text! This analogy may help you remember the salient aspects of the mitotic phases.

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168 Unit 2: Is it all in the Genes?

Molecular processes during Mitosis In order to carry out mitosis, just before the start of cell division, DNA doubles itself during the S (synthesis) phase. This doubling process is termed replication and fol- lows a series of specified steps. First, during the unwinding phase, the vertical strands unwind beginning at a certain sequence of bases called the Initiation sequence. Helicase is the enzyme that untwists the double helix so replication can occur. This untwisting is much like a zipper unzipping. This step exposes bases within the DNA so that new bases may be added onto the exposed strands by special enzymes called DNA polymerases. In the next rebuilding phase, the exposed base strand allows a new layer of nucleotides to form along the existing base sequence. Each of the single strands becomes a double strand. To accomplish this, another set of enzymes, DNA polymerases, carry comple- mentary bases to the exposed regions, so that, for example, whenever an A is exposed a T binds to it and whenever a G is exposed a C binds to it. In this way, each new strand contains an old set of nucleotides and a new set of nucleotides. The end result is two double-stranded DNA molecules, both identical to its parent strand: half original material and half newly placed by the rebuilding phase. Because of this half-new and half-old DNA structure, this process of replicating DNA is termed semi-conservative. See Figures 5.11 and 5.12, which illustrates this doubling of DNA.

Energy needed to add bases during replication is obtained by DNA polymerases through hydrolyzing nucleoside triphosphate (a relative of ATP). DNA polymerases move along the DNA molecule in a certain way. DNA polymerases add bases in a certain direction on the DNA molecule, as shown in Figure 5.11. Notice that the unzipped DNA looks like a fork, – in fact it is called a replication fork – with two sides of the molecule exposed for adding bases. DNA polymerases can add nucle- otides only where there are already existing nucleotides in place. A primer is laid down to start the process. This is a segment of RNA molecules 10 nucleotides in length. DNA polymerase in Figure 5.12 recognizes this sequence and begins add- ing nucleotides. In this part of the replication fork, the DNA polymerase moves

initiation sequence

A sequence of bases that starts the unwinding of DNA during replication. helicase

The enzyme that untwists the double helix so that replication can occur. DNA polymerase

Special enzymes that add new bases onto the exposed DNA strands. semi- conservative model

A mode by which DNA replicates as half-new and half-old DNA.

Nucleoside triphosphate

A molecule that contains a nucleoside bound to three phosphates.

Replication fork

Molecules with both its sides exposed for adding bases.

(a) (b)

Figure 5.11 Semi-conservative model of DNA showing replication fork. a. During replication DNA poly- merase lays down a new set of nucleotides in a replication fork. From Biological Perspectives, 3rd ed by BSCS. b. The new neucleotide strand is complementary to an old strand.

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smoothly to produce newly added segments. As the fragments are laid down, primers are detached and DNA ligases combine the segments of DNA to create one smooth DNA molecule.

There are approximately six billion base pairs in our 46 chromosomes. In order for replication to occur, multiple areas are untwisting at any one time, with nucleotides being added constantly and rapidly. While there are so many possibilities for mutation or error, only 1 in 100,000 errors actually occur. While this may appear like a small number, when considering how much DNA a cell has, three billion base pairs, errors would result in more than 120,000 mistakes every time a cell simply divides. To rem- edy these problems, there are about 50 different types of DNA repair enzymes, which remove mutated nucleotides and replace them with correct complementary ones. DNA polymerases and DNA ligases both work together with DNA repair enzymes to create a new strand of DNA, moving along the DNA molecule at a speed of about 20 base pairs per second in humans. Of course, errors in replication still happen, leading to mutation and gene changes. Joyce Carl experienced the effects of these mutations on select genes controlling albinism.

DNA ligase

A type of enzyme that joins DNA strands together.

Figure 5.12 DNA replication showing a chemistry clip of replication fork adding complementary bases. From Biological Perspectives, 3rd ed by BSCS.

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Another example of a disorder resulting from an error in DNA replication is found in the English peppered moth, Biston betularia (Figure 5.13).There are two varieties of this peppered moth: light- and dark-colored. It was shown that the dark color arose because of a rare but recurring mutation in some moths. During the period of industri- alization in England in the 1800s, dark moths comprised almost 98% of the population in the city of Manchester, which was known for its sooty conditions. Why might this change in variety proportions have occurred? Studies by H.B.D Kettlewell showed that darker moths thrived because they blended better with their polluted surroundings than lighter colored moths. He hypothesized that lighter moths stood out to predators. This is an example of how mutations can help some members of a species to survive. Hav- ing a genetic variation in populations because of mutations is healthy for any species’ survival. Changing environmental conditions sometimes allow survival of at least some individuals.

Why Go through it all? prokaryotic Cell Division is More simple Bacteria do not go through the many steps of mitosis to reproduce. Instead, they divide using a process called binary fission. Genetic material is in a circular form in prokaryotes, within what is termed a circular genome. Prokaryotes do not have a nuclear envelop to protect their genetic material. Thus, DNA in prokaryotes is called “naked DNA,” without a nuclear covering. Circular, naked DNA divides to form two new circular DNAs, each attaching to two different areas of the cell membrane. As a prokaryotic cell grows, it pulls the genetic material to opposite ends of the cell. Cytokinesis then occurs, forming the physical separation between the cells. Separate, smaller circles of genetic material, called plasmids, carry information for specific activities within a cell. Plasmids repli- cate independently of the circular genome, also moving into new daughter cells.

How does genetic diversity get maintained in prokaryotes when parents are identical genetically to offspring? Mutations during replication are a primary source of difference between prokaryotes. Diversity is also promoted through bacterial genetic exchange or sometimes called “bacterial sex” because DNA is getting transferred between organ- isms. In this process, genetic material is exchanged through pili, or hair-like structures, that connect two bacterial cells through which DNA is exchanged.

Prokaryotes have been on Earth almost since its origin – for over 3.9 billion years. Prokaryote genetic resilience is astounding. Bacterial diversity is maintained through

Binary fission

The process by which a cell divides directly in half.

Circular genome

Genetic material in a circular form found in prokaryotes.

Figure 5.13 A light variation of Biston betularia, the English peppered moth.

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mutation because there are many cell divisions and many chances for errors and muta- tions. Consider that, on average, it takes a generation, or 25 years, for humans to repro- duce. It takes bacteria only minutes to form new organisms. This rapidity in reproduction allows for more chances for mutation; and thus more chances for genetic variation. In our story, Joyce suffers for her genetic variation, but without it, any species would die off quickly, unable to withstand changing environmental conditions. Mutations can range from being unnoticed to creating monstrous results, depending on how the gene sequence is transmitted to the rest of the cell.

Dna is the Universal language While Watson and Crick’s model of DNA established it as the hereditary material and explained the mechanism for replication, it did not explain how the instructions are carried out. The way the genetic code is read and expressed in living systems will be discussed in this next sections. In short, we need to answer the question: “How are genes expressed?”

Genes control the functions of any cell by giving directions or orders to carry out. The orders are found in its nucleotide sequence that is read to allow the genes to be expressed. How does DNA accomplish this? The four types of nucleotide bases in DNA form a nearly infinite number of possible combinations to create a language of information to transfer from DNA to cell structures. DNA has complex instructions for building cells and organelles for every type of organism. All of life’s diversity depends on this molecule and the many possible arrangements of nucleotides that create its language.

After Watson and Crick’s model described its structure, DNA was studied to deter- mine what it actually does. Linus Pauling reasoned that disease was due to differences in chemicals between normal and afflicted people. In particular, Pauling studied hemoglo- bin protein differences between people with sickle-cell anemia, carriers for the disease, and normal individuals. Sickle-cell anemia is a disease that leads to abnormally shaped red blood cells, poor oxygen carrying capacity, and a host of complications such as blood clots and organ damage. Carriers for sickle-cell anemia only have one copy of the sickle cell gene and sometimes show symptoms, but usually only under extreme circum- stances (e.g., severe dehydration, physical exhaustion, or high altitude). Those afflicted with the disease carry both copies of the sickle cell genes.

Pauling used electrophoresis, a process of separating organic materials based on their electric charges, and found a difference in hemoglobin’s proteins: the hemoglobin of normal people carries a stronger negative charge than that of sickle cell sufferers (Fig- ure 5.14). The hemoglobin of sickle-cell carriers has a charge somewhere in between. This was the basis for determining that genes affect protein structure. Thus, Pauling deduced that, because proteins are found everywhere in the body doing so many varied things, DNA must somehow affect protein structure. Pauling’s ideas eventually led to the discovery that our genes give directions to make up to 2 million different proteins. In the example of sickle-cell anemia, some years after Pauling, Vernon Ingram showed that there was only one difference in proteins between sickle cell hemoglobin and normal hemoglobin. One in 300 amino acids was changed, which was enough to reorient the entire structure leading to a sickle-shaped red blood cell.

The carrier’s (and an afflicted person’s) hemoglobin shape was later determined to confer some degree of immunity to a tropical disease called malaria. Malaria is an infectious disease spread by mosquitos carrying a parasite that invades red blood cells and reproduces in them. It causes flu-like symptoms, ranging from fever and chills to

sickle cell anemia

A disease that leads to abnormally shaped red blood cells, poor oxygen carrying capacity, and a host of complications such as blood clots and organ damage.

Malaria

An infectious disease spread by mosquitos carrying a parasite that invades red blood cells and reproduces in them.

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172 Unit 2: Is it all in the Genes?

respiratory problems, coma, and death, if left untreated. How does an immunity to malaria manifest? Basically, the alternate hemoglobin shape of sickle cell gene holders (carriers and afflicted individuals) causes enough 3-D changes in their red blood cells to pre- vent the malarial parasite from getting into red blood cells and dividing. Plasmodium is the protist that infects human red blood cells, causing this malady. Possessing gene copies for sickle-cell anemia prevents Plasmodium from entering the human red blood cell.

There are more than 300 million cases of malaria each year, and it kills about 1 million people in Africa and Asia annually, according to the World Health Organization 2010 World Malaria Report. Sickle-cell carriers have 1/10th the likelihood of contract- ing the most dangerous form of cerebral malaria. Thus, being a carrier for sickle-cell anemia in tropical areas, where the disease is most likely to spread, is beneficial. The carriers’ benefits come at a price though, because some individuals in the population are going to have sickle-cell anemia. This is an example of how a harmful mutation, such as sickle-cell anemia, can persist in a population: when there is a benefit to survival, such as in this case (in warm areas affected by the disease), the mutation will continue to be expressed for thousands of years.

What Do proteins Do? As discussed in Chapter 2, proteins perform almost every aspect of an organism’s means for maintaining an existence. Feel your skin – it is keratin protein that protects you. Insects use chitin protein for their protection. An analysis of our hormones, such as insulin, shows a variety of very specific proteins perform very specific functions. Pro- teins are also enzymes, carriers of oxygen such as hemoglobin, and make up half of cell membranes; they compose hair, hold cells together, receive hormones and chemicals for cells, and move muscles, to name a few functions. Joyce Carl had albino skin coloration because she could not produce the protein melanin, a skin-color molecule which makes cells a darker shade. Proteins therefore express the essence of being alive because they carry out both structural and functional aspects of life functions. Figure 5.15 shows the varied roles that proteins play in living systems.

Melanin

The pigment that gives color to human eyes, hair, and skin

Figure 5.14 Genetic differences between normal and sickle-cell gene sequences lead to changes in red blood cells shown above, with the abnormal red blood cell shaped like a sickle. Only one base pair change (from GAG to GUG) between normal and sickle cell DNA causes one amino acid change and, thus, the disease. From Biological Perspec- tives, 3rd ed by BSCS.

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Gene expression: how proteins are Made Gene expression is defined as the ability of a gene to carry its information to the rest of a cell and perform its directives. The way our genes accomplish gene expression is to make the variety of proteins described in the previous section. Gene sequences produce proteins to carry out orders found in the genetic code.

However, not all genes are made into protein. Only about 5% of our genes give rise to proteins. In fact, more than half of our genes contain information that simply repeats thousands to hundreds of thousands of times, with no information to be passed on. These sequences are noncoding, or do not produce proteins. These nucleotide sequences do not become expressed as proteins because they are spliced out. In other words, they do not have a chance to code for proteins and therefore influence an organism’s traits. When genes do get expressed, however, it results in the production of protein and therefore has the potential to affect organisms’ characteristics.

Thus, while the totality of our genes comprises our genotype, or genetic make-up, only those genes leading to or coding for a protein will result in our observable char- acteristics. Our protein make-up results in our observable traits, or the “way we look,” which is termed our phenotype. Joyce Carl’s phenotype was albinism, but her genotype led to the condition. How are there changes between the genotype and the expressed phenotype? Simply put, in order to be expressed, a gene must be able to code for a pro- tein. If Joyce’s gene mutation for albinism had occurred on a portion of genes that do not code for proteins, she would not have developed albinism.

The production of proteins from DNA begins in the nucleus of eukaryotes. Eukary- otic organisms have cells with chromosomes protected by a nuclear membrane. DNA remains protected in the cell, which prevents potential damage to those chromosomes. Because we know from Chapter 3 that ribosomes make proteins, where do you predict the message will be sent from the nucleus? Yes, messages are sent from the nucleus to the ribosomes to express a message into a protein form.

Gene expression

The ability of a gene to carry its information to the rest of a cell and perform its directives.

Genotype

The genetic makeup of a cell.

Phenotype

The observable traits of an organism.

Figure 5.15 Varied roles of proteins in living systems a. Roles of proteins in living sys- tems. b. How do proteins affect our appearance? From Biological Perspectives, 3rd ed by BSCS.

(a)

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174 Unit 2: Is it all in the Genes?

Molecules that carry a message from the nucleus are called messenger RNA or mRNA. They are single strands of nucleotides that carry coding sequences for strings of amino acids (forming a polypeptide). As described in Chapter 2, amino acids are the basic subunit of all proteins. This process of information transfer from a gene sequence to a polypeptide requires two steps. The first, transcription, moves a message from the nucleus to the cytoplasm, and the second, translation, reads that instructions forming a chain of amino acids that reorient and constitute a protein:

1) Transcription: DNA ➔ mRNA 2) Translation: mRNA ➔ Polypeptide à Protein

This two-step process is so important to understanding biology that it is known as the Central Dogma of Biology (Figure 5.16). It explains how inherited material gives rise to all our unique structures, functions, assets (like a pleasant personality or a high intelli- gence), and liabilities (like disease).

With 20 different types of amino acids within a living cell, innumerable combina- tions are possible for making any type of protein needed for life functions. How can so many amino acids be made from a set of only four types of bases? The answer is that it takes a sequence of three bases in the DNA and mRNA to “code” for a single amino acid. When DNA codes for an amino acid, it has a specific set of instructions which match to the production of an amino acid. Consider the DNA sequence, AAA (three adenine bases) that codes for UUU (three uracil bases) on the mRNA molecule. UUU delivers a phenylalanine amino acid to a growing polypeptide. The flow of information is given as a simple equation below:

AAA ➔ UUU ➔ Phenylalanine

The set of instructions, in the case above AAA is a sequence of three bases (car- ried by its respective nucleotides), called a triplet sequence on DNA. Its corresponding

RNA

A nucleic acid present in living cells.

mRNA

Are molecules that carry a message from the nucleus.

Transcription

The first step of gene expression in which information in a DNA strand is copied into mRNA by RNA polymerase.

Translation

The synthesis of protein from the information contained in a molecule of mRNA.

Central Dogma

A theory that explains how inherited material gives rise to all our unique structures, functions, assets, and liabilities.

Figure 5.16 Central Dogma of Biology. Gene expression is a two-step process in which DNA is transcribed into RNA which is then translated into proteins. From Bio- logical Perspectives, 3rd ed by BSCS.

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sequence on mRNA, in our case given, UUU, is termed a codon. Figure 5.17 shows the combinations of codons that code for their respective amino acids. A specific codon leads to a particular amino acid placed onto a protein that forms in the process of a gene’s expression. Thus, the order of nucleotides in the DNA and resulting mRNA strands determine the combination of amino acids in a protein for which the genetic material codes. There are 64 possible codons, and 61 are known to code for the 20 amino acids in existence. The other 3 codon triplets serve as “start” and “stop” signals for the process of protein production. Neither of these code for amino acids except for the start codon, AUG, which also serves to code for the amino acid methionine. The sequence of amino acids within a protein is important because it determines the three-dimensional structure, orientation, and function of respective proteins. Use the chart in Figure 5.17 to trace the nucleotide sequences that produce their specified amino acids. Which amino acid develops from an original triplet DNA sequence, CCT?

If you answered glycine, you correctly traced the origins of the amino acid to its DNA code. Most of the time, amino acids are correctly coded for by their triplet and codon sequences.

At times, however, if one specific amino acid is misplaced, an incorrectly formed protein results. Such a scenario may lead to disease. To illustrate, consider our earlier

Codon (triplet)

Normal genetic code in which a sequence of three nucleotides codes for a specific amino acid.

Figure 5.17 Genetic code (for amino acids) table. This table shows the genetic code in the mRNA codes for specific amino acids, during translation. Start codons and stop codons do not specify a particular amino acid, instead they signal the cell to start or stop translation. From Biological Perspectives, 3rd ed by BSCS.

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176 Unit 2: Is it all in the Genes?

exampler sickle-cell anemia is due to a single amino acid error. A change in a single base (thymine to adenine) in the DNA strand leads to amino acid number 6 switching from glutamic acid to valine. This single amino acid difference leads to the structural changes described earlier in the hemoglobin.

The goal of transcription is to copy a sequence of nucleotide bases correctly into an mRNA molecule. While mRNA is somewhat different than DNA (see Chapter 2 to review the differences), it is able to match up to the DNA molecule to produce a comple- mentary set of nucleotides. There are three phases of transcription: Initiation, Elongation, and Termination (Figure 5.18b). The first step in initiation is to again, as in replication, unwind the DNA molecule. Instead of DNA polymerase and helicase accomplishing this, RNA polymerases bind to a specific sequence to unwind the DNA at certain sites and start transcription. These special sites are called promoter sites, which are composed of a series of base sequences. This region occurs at the beginning of each gene and ensures that the mRNA is made from that point forward. RNA polymerase moves from the pro- moter and along the DNA molecule, adding complementary bases in the elongation phase. Bases are added in the same manner as during replication, with one exception: Uracil, a complementary base is found in mRNA (which replaces thymine), is matched with adenine bases on the DNA molecule to produce mRNA. For example, if a sequence of DNA being copied is

ATTGCCACC

The mRNA sequence will have a complementary strand of

UAACGGUGG

Again, note that it is the same type of complementary base pairing as in replication, but uracil is found in RNA in the place of thymine. The other base pairings remain the same. Eventually, RNA polymerase will reach a sequence of DNA that tells it to stop, called a termination sequence. This is the end of the gene, and RNA polymerase detaches from the DNA strand. During the termination phase, mRNA is released from the DNA mol- ecule and the separated DNA strands reform into a double helix. A cap and tail is then added to the mRNA molecule before it leaves the nucleus for protection. This is called RNA processing and protects the mRNA information, much as a cover protects a book. The mRNA makes its journey out of the nucleus through nuclear pores because ribo- somes are found within the cytoplasm. It was at this point in Joyce Carl’s transcription process that her albinism first became expressed. Joyce probably had Oculocutaneous albinism type I, which results in the transcription of a mutated gene on chromosome 15. The code was brought out into the cytoplasm to be made into protein.

If her gene had not been transcribed, Joyce would not have developed albinism. Some genes are not expressed into proteins causing organisms’ traits. Eukaryotic cells process these noncoding sequences, called introns, by splicing them out and leaving only coding sequences, exons. The message then gets sent through the pores of the nuclear envelope into the cytoplasm to be “read” and made into proteins.

reading the Message: translation When mRNA leaves the nucleus through nuclear pores, it attaches to ribosomes. Like workers in a mini-factory, ribosomes work to read the message on mRNA coming out of the nucleus. There are many “workers,” each with a specialized task in the transfer of genetic information on mRNA into the amino-acid sequence found in all proteins. Translation is also composed of three phases, termed Initiation, Elongation, and Ter- mination, the same names used for the phases of transcription (Figure 5.18b). We’ve seen that codons on mRNA either give start or stop directions or code for amino acids.

elongation

One of the three phases of transcription in which nucleotides are added to the growing RNA chain.

Termination

The phase in which RNA polymerase will reach a sequence of DNA that tells it to stop.

RNA processing

The process in which cap and tail is added to mRNA before it leaves the nucleus (for protection).

intron

A nucleotide sequence removed by RNA splicing.

exon

A segment of RNA or DNA that contains information coding for a protein.

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Amino acids are brought into the ribosome because they match complementary codons on the mRNA molecule. Many amino acids have more than one codon on the mRNA that draws it into the ribosome. For example, using the genetic code table in Figure 5.17, the amino acid alanine is shown to have four different codons that code for it: GCA, GCC, GCG, and GCU. The codons draw amino acids to the mRNA to form proteins.

However, mRNA does not directly produce amino acids. It requires a host of other “workers” to make proteins. Transfer RNA (tRNA) molecules are shaped like a clo- ver, carrying specific amino acids on one side of their shape and on the other, binding with specific sequences on the mRNA molecule. In this way, amino acids are brought in to match the sequence found on the mRNA molecule. The process occurs with the assistance of the ribosomal RNA (rRNA). Ribosomes are composed of two subunits consisting of rRNA. Ribosomal subunits have specific shapes to hold the mRNA strand while amino acids are added together. See Figure 5.18 for the structure of tRNA, rRNA, and mRNA.

tRNA

Small RNA molecules that carry amino acids to ribosomes for protein synthesis.

rRNA

RNA component of ribosome.

Figure 5.18 a. Methionine tRNA starts the process of translation, shown as the first to arrive at a ribo- some in the figure. Then, other tRNAs bring amino acids to ribosomes during translation. During trans- lation, a string of amino acids (forming a protein) is made along the surface of a ribosome. From Biological Perspectives, 3rd ed by BSCS. Reprinted by permission. b. The steps of transcription: the movement of RNA polymerase along a DNA molecule, making mRNA. From Biological Perspectives, 3rd ed by BSCS.

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ribosome (b)

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Figure 5.18 (continued)

Initiation of translation begins when start codons (always AUG) signal the location in the mRNA to begin translation. A tRNA carries a matching, complementary sequence to the start codon. A start codon on the mRNA is always AUG, which matches to a par- ticular tRNA carrying methionine. Figure 5.18a shows that the first amino acid in the sequence being made is met (methionine) Every tRNA has a sequence of bases on it called an anti-codon, which matches with the bases found on an mRNA. UAC is the start anti-codon on the first tRNA to bring an amino acid, methionine, to the ribosome. UAC matches (or is complementary with) the AUG start codon on mRNA.

As anti-codons on tRNAs match up with mRNA bases, amino acids are brought to the ribosome. Matching tRNAs brings one amino acid after another to the mRNA sequence. Each amino acid is linked to the next via a peptide bond. This process of elon- gation continues, adding amino acids alongside the mRNA information strand.

• Dehydration synthesis, discussed in Chapter 2, links macromolecule subunits together to form larger amino acid chains.

Enzymes on the ribosome catalyze the process, removing water to form peptide bonds between amino acids. The growing polypeptide chain continues to elongate until tRNA

Anti-codon

A sequence of three nucleotides in transfer RNA molecule.

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Chapter 5: Molecular Genetics 179

reaches a stop codon of several types: UAA, UAG, UGA that signals the end of transla- tion. Stop codons do not code for any amino acids. At this point along the mRNA, called termination, tRNA, the ribosome, and mRNA detach from each other; the amino acid polypeptide chain is released simultaneously. It is a multipart process in which many specialized “workers” come together for a moment, guided by a message far away in the DNA, to make a whole protein. Figure 5.19 depicts the many molecular players involved in transcription and translation, forming proteins.

Once released from the ribosome, the polypeptide chain reconfigures and adopts its unique three-dimensional conformations based on the chemistry of the amino acid sub- units. Large molecules may form, such as the hemoglobin shown in Figure 5.20.

We began the chapter describing Joyce Carl’s struggle with albinism. Essentially, we now know that albinism is a problem with the making of enzyme proteins that produce the pigment, melanin. Melanin gives skin, eyes, and hair their color and protects the skin from damage by ultraviolet (UV) light. It shields the nucleus of cells to prevent damage to chro- mosomes and the vitally important DNA sequence within the nucleus. This is why sunlight is so dangerous for people like Joyce, because the sun’s rays can mutate skin cells to make them cancerous. Albinism is originally caused by a mutation on either chromosome num- ber 5, 9, 11, or 15, each leading to its own type of albinism. This mutation is transcribed and then translated into precursor enzymes that are unable to lead to normal melanin pig- ment production. In Joyce Carl’s form of albinism, the most common in Sub-Saharan Afri- can and African-Americans, hair is yellow or ginger in color and eyes are often gray-blue. With limited melanin, skin often freckles with moles over time due to sun exposure.

In fact, skin color on the whole is due to only 10 different genes out of our total of up to 25,000 genes. Skin color evolved because of environmental benefits to individuals in the past. To illustrate, sunlight can be devastating for our skin’s health, as in the case of the Joyce Carl or as you may have learned if you experienced a serious sunburn. UV light has been shown to deplete folic acid from the skin. Folic acid is a very important nutrient in a fetus’s brain and spinal cord development. Thus, in the distant past in a world lacking nutrition, melanin provided the needed protection so that folic acid could be spared for proper fetal development. Sunlight has been shown to deplete folic acid. In sunnier climates, it was beneficial to have darker skin not merely to protect against skin cancer, but to protect folic acid from sunlight and thus retain needed folic acid for the growth and development of the next generation. Thus, evolution favored darker skin tones in climates with more sunlight and thus, the evolution of darker skin colors.

On the other hand, sunlight facilitates the body’s production of vitamin D. Because melanin blocks UV light, less vitamin D synthesis takes place in people with darker skin tones. Fifty thousand years in the past, therefore, when food choices were limited, it would have been better to have lighter skin to absorb sunlight and produce vitamin D. However, because skin color is a result of only about 10 different genes, there are many combinations and color types. A random shuffling of genes can result in a set of very light color genes for one fraternal twin of a pair and a set of darker color genes for the other. What determines “race?” and Does race even exist? are questions to ask when classifying people based solely on the 10 genes of skin color.

Gene regulation Not all our genes are expressed in our phenotype. Cells in the kidney do not express hair color, for instance – there is no need. Cells produce only what is necessary, as we’ve seen elsewhere in the text. In fact, genes are active only 5–10% of the time in a normal living cell. The ability to shut certain genes off and turn some genes on, like a light switch in a room, is termed gene regulation. Overproduction of materials is unnecessary.

Folic acid

A water-soluble vitamin and a very important nutrient in a fetus’s brain and spinal cord development.

vitamin D

A fat-soluble vitamin that promotes that is essential for the absorption of calcium.

Gene regulation

The ability to shut certain genes off and turn some genes on.

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180 Unit 2: Is it all in the Genes?

Figure 5.19 Steps in the process of translation: initiation, elongation, termination (a top) and tran- scription (at the bottom).

5' end

A A

C C

G G

Messenger RNA (mRNA)

Transfer RNA (tRNA)

AA AA

Amino acid

Hydrogen bond

Growing protein chain

Direction of translation

G C

P P

Sugar Sugar

SugarSugar

P P

Sugar SugarA

Nuclear DNA

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ct io

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3' end

Nucleus

Each set of three mRNA bases is a codon which specifies one amino acid

TRANSLATION

TRANSCRIPTION

NH2

CO OH

Adenine

Guanine

Cytosine

Uracil

Symbols for organic bases

Ribosome

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Chapter 5: Molecular Genetics 181

Figure 5.20 Hemoglobin molecule. Three-dimensional shape gives hemoglobin its function. There are four polypeptide chains together forming a quarternary structure. The specific orientation of amino acids allow for oxygen carrying “heme” groups to sit within the molecule and hold enormous amounts of oxygen. The unique shape of hemoglobin is shown in the image. In the next chapter, this shape is the impetus for the suspense story. From Biological Perspectives, 3rd Edition by BSCS.

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182 Unit 2: Is it all in the Genes?

Remember, the cell is efficient and cheap. It spends and produces only when it is neces- sary, a theme of this text.

There are two primary mechanisms for gene regulation. First, the promoter region, discussed earlier in our transcription section, may be turned off. For example, the addi- tion of chemical groups to DNA may prevent RNA polymerase from binding to DNA. Second, DNA is wrapped around histone proteins that cover the promoter region, as shown in Figure 5.21. When histones are chemically modified to unwind DNA strands, the gene can be expressed. However, under tight coiling conditions, without access to the promoter site, no transcription and thus no translation can occur.

errors in Gene regulation: a Focus on Cancer At times, cells divide uncontrollably when gene regulation fails. Unchecked and unreg- ulated by their own genetic “stop” mechanisms, cells grow out of control. Abnormal, uncontrollable cell division results in a tumor, or an abnormal growth of cells. The most common cause of tumors is cancer. Cancer is caused by gene changes that prevent nor- mal rates of mitosis. Cancer is a complex of over 200 related diseases and is a leading cause of death in the world.

Cancer was first thought to be of genetic origin by Theodore Boveri, who studied pedigrees of families with cancer and noted emerging patterns of inheritance. He thus proposed that normal cells become cancerous when their chromosomes become altered in some way to prevent the usual mechanisms of control over mitosis. Today, his research is shown to be correct in its assumptions about cancer.

There are four major characteristics of cancer cells:

1) A loss of contact inhibition, which is the cell’s normal ability to come into con- tact with its neighbors while dividing and inhibit its growth based on the limited spacing around it;

2) Dedifferentiation, which is a loss of the specialized functions that normal cells perform. For example, a normal kidney cell will participate in filtering materi- als from the blood, but a cancerous kidney cell will not filter or function like a kidney cell;

3) Loss of cellular affinity, which keeps the cell with cells that are histologically similar to itself. A normal kidney cell, when mixed in a petri dish with liver cells, will tend to migrate to other kidney cells. However, cancerous kidney cells lose this affinity and attach to liver cells instead of other kidney cells. This is a most dangerous characteristic because it allows cancer cells to metastasize, or spread to other parts of the body. When a cell is determined to be capable of spreading it is termed malignant. Malignant cells enter either the lymphatic sys- tem or the blood stream to migrate to other parts of the body and grow. It is this growth that gets in the way of other organ functions and leads to serious health consequences and sometime death; and

Cancer

A tumor caused by an uncontrolled division of cells.

Contact inhibition

Cell’s normal ability to come into contact with its neighbors while dividing and inhibit its growth based on the limited spacing around it.

Dedifferentiation

Is the loss of the specialized functions that normal cells perform.

Metastasize

The process in which cancer cells spread to other parts of the body.

Malignant

The ability of a cell to spread.

Figure 5.21 A histone protein is wrapped in DNA.

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Chapter 5: Molecular Genetics 183

4) Immortality – Cancer cells may live for an eternity if given the right conditions such as nutrients and water. They do not age as other cells do; instead an enzyme called telomerase rebuilds their DNA ends, or telomeres, so that the cells may replicate forever. In normal cells, telomeres wear out after about 100 DNA rep- lications, at which point the cell can no longer divide, and it dies.

Is simply injecting telomerase the fountain of youth? Preliminary studies show injec- tions of telomerase into experimental animals, instead of leading to longevity, caused tumorogenesis, or increased tumor formation, and thus premature death. In other words, injecting telomerase caused increased cancer rates in animals. This dampened the enthu- siasm for telomerase as a fountain of youth. The inheritance of cancer is under some degree of genetic control. Cancer may strike at any age and any socioeconomic class. For example, Hollywood star Christina Applegate (see Figure 5.22) fought breast cancer in her thirties and is now cancer-free.

summary The chapter began with the plight of a young person, Joyce Carl, who endured social discrimination and physical harm as a result of the natural processes described: tran- scription, translation, mutation, and gene expression. The processes of replication and mitosis serve to produce new, identical cells. In order for genetic material to direct the activities of a cell, it is transcribed and translated into proteins. These proteins then act within a cell to carry out its activities. With a change in melanin production, for exam- ple, our chapter story shows that lives are changed within our society. The processes described in this chapter yield our many characteristics that enable life functions. Those traits will be described in more detail in the next chapter. It is the hope that greater knowledge about genetics and its promise for improving human health will lead to a more understanding world.

Telomerase

An enzyme that rebuilds the DNA ends of cancer cells.

Telomere

A compound structure found at the end of a chromosome.

BioeThiCs Box 5.2: iMMoRTAliTy oF helA Cells – siNCe 1951 AND GoiNG sTRoNG!

A best-selling book by Rebecca Skloot, The Immortal Life of Henrietta Lacks, chronicles a unique 1951 case in which a woman’s cervical cancer cells were harvested and grown, although without her knowledge. The patient, Henrietta Lacks, died of cancer the same year, but given nutrients her cancer cells are still kept alive in labs around the world, where they are used in experimenta- tion and observation. Recently, these HeLa cells (named after the patient from whom they were drawn) were studied to determine a relationship between human papilloma virus and cervical cancer. This resulted in a vaccine to prevent transmission of cervical cancer.

Of course, cancer cells do die when the host organism dies, because they lack nutrients from the body. Cancer cells are subject to the same kinds of needs as any other cell, but they look different. Their structure is different, mitosis occurs more frequently, and dedifferentiation is obvious. The heirs of Henrietta Lacks are in court to determine the legal ownership of HeLa cells and, of course, who benefits financially from HeLa’s scientific results. The legal results remain to be seen.

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184 Unit 2: Is it all in the Genes?

Figure 5.22 Actress Christina Applegate was diagnosed with breast cancer in 2008.

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Our chapter starts with the story of human albinism in Joyce Carl. However, albinism can occur in animals, including whales. Years before classic author Herman Melville wrote his fictional work, Moby Dick, about a great white whale, whalers were captivated by another great white … “Mocha dick.” This large albino sperm whale was named after the Chilean island of Mocha in the Pacific. From 1810 through the 1830s Mocha Dick had numerous encoun- ters with whalers – attacking and damaging numerous ships, leaving some men dead. Mocha Dick was likely not the only albino whale in the sea, but he was certainly a notable inspiration to a classic tale. Our next chapter begins with a story about vampirism, also an inherited characteristic, with a history long ago emanating from parts of Eastern Europe.

CheCk OUt

summary key points

• Differences in traits may lead to serious social discrimination issues. • DNA is the hereditary agent of transmission. • Genes are sections of DNA that contain instructions for making proteins. • Replication is the way DNA divides itself. • Transcription is the way DNA is made into messenger RNA. Translation is the way messenger RNA

is made into proteins. • Skin color evolved due to the benefits and drawbacks of the environments of the times. • Cancer is an inability to regulate the genetics of mitosis.

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Chapter 5: Molecular Genetics 185

Albinism Adenine Anaphase Animalcules Anti-codon Binary fission Cancer Cell cycle Central Dogma Chromosomes Circular genome Codon (triplet) Complementarity Contact inhibition Cytokinesis Cytosine DNA DNA Polymerase DNA ligase Dedifferentiation Deoxyribose Elongation Exon Folic acid G1 phase G2 phase Gene Gene expression Gene regulation Genotype Guanine Helicase Histone

Initiation Interphase Intron Malignant Malaria Metaphase Metastasize Melanin Molecular genetics Nucleotide Nucleoside triphosphate Phenotype Prophase Replication fork Ribose RNA RNA processing mRNA tRNA rRNA S phase Semi-conservative model Sickle-cell anemia Telomerase Telomere Telophase Termination Thymine Transcription Translation Uracil Vitamin D

key TeRMs

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Chapter 5: Molecular Genetics 187

Multiple Choice Questions

1. How many genes determine skin color in humans? a. 10 b. 100 c. 1,000 d. 1,000,000

2. A person is often judged by his or her appearance. Which most affects how a person is perceived in society? a. exons b. genotype c. phenotype d. complement

3. Miescher, Griffith, and Avery each sought to explain heredity based on Mendel’s laws. Which did they each focus upon? a. pea plants b. dominance c. recessiveness d. chemicals

4. Which of the following are the same in every DNA molecule? a. ribose b. ligase c. polymerase d. phosphate

5. Which portion of the nucleotide is most important in transmitting information? a. deoxyribose b. ribose c. phosphate d. adenine

6. Which occurs when a mismatched nucleotide is expressed in a gene sequence? a. a changed protein b. a changed mRNA c. a mutation d. all of the above

7. How many amino acids are produced from a gene sequence containing the follow- ing bases: TTAACGCCCCTA. Assume that all of the genes are expressed as amino acids and no noncoding or start/stop sequences are included. a. 1 b. 4 c. 12 d. 24

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188 Unit 2: Is it all in the Genes?

8. DNA polymerase serves to: a. lay new RNA nucleotides b. lay new DNA nucleotides c. fuse DNA fragments d. fuse RNA fragments

9. Which best describes the genetic cause associated with sickle-cell anemia? a. a missing piece of chromosome 14 b. an elongated piece of chromosome 14 c. a mutation from thymine to adenine d. a mutation from valine to glutamic acid

10. Which most likely linked to evolutionary changes in melanin production over the past 50,000 years of human evolution? a. folic acid b. Plasmodium frequency c. complementarity of bases d. cell affinity

short answers

1. Draw a molecule of mRNA derived from the DNA sequence: TTAGGCCACCTC.

2. List three differences between a strand of DNA and a strand of RNA.

3. Draw a diagram of the Watson and Crick double helix and label its parts, including deoxyribose, phosphate, adenine, guanine, cytosine, and thymine. Show hydrogen bonds using dotted lines.

4. Name two enzymes that are needed for replication. Explain the role of each enzyme in doubling the DNA.

5. What is meant by the term “semi-conservative?” Use a drawing with colors to explain how DNA is made using this term.

6. Explain the process of DNA transcription to a friend. What are the main results of the process? Do the same for translation.

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Chapter 5: Molecular Genetics 189

7. Place the following terms in the correct order, starting from the beginning to the end, tracing the flow of materials through the central dogma: Promotor mRNA DNA DNA polymerase DNA ligase rRNA tRNA Methionine RNA polymerase

8. Transcribe the following sequence of DNA: TTAACGCC

9. Which of the following sequences cannot exist for an mRNA? a. ATTGCC b. UTTCCT c. AAAATT d. CCCCCC Explain your answer above.

10. Antibiotics are used to kill bacteria by stopping the ribosome from functioning. Based on the central dogma of biology, why is this so deadly for bacteria?

Biology and society Corner: Discussion Questions 1. What was Watson and Crick’s main purpose for making a model of DNA? How

does it lead to the information given in this chapter? Would we have still been able to develop this chapter without their model of DNA? Why or why not?

2. Based on the readings in this chapter, is there such a thing as “race” in humans? Explain your answer.

3. Who should own the rights to cells harvested from people during medical proce- dures? Why?

4. What could be done by the Tanzanian government to prevent discrimination of Afri- can albinos in a culture which holds beliefs that endanger their lives? How can African albinos best improve their social integration into society?

5. Write a plan to help African albinos, who are in fear for their lives, cope with the existing social discrimination. What are four ways albinos can improve the quality of their lives?

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190 Unit 2: Is it all in the Genes?

Figure – Concept Map of Chapter 5 Big Ideas (Below is a sample of a concept map for this chapter - You may draw your own in the box provided above to help you make your personal connections)

known as

Genes organized into

divide by

confers immunity to

errors

how you appear

genetic errors lead to make up Mutations

leads to such as characteristics

Central Dogma of Biology

genotype

phenotype

DNA RNA Protein

transcription translation

Cancer

Replication

Albinism

Sickle Cell Anemia

Loss of Cell Affinity

English Peppered MothsEnglish

Immortality

Dedifferentiation

Loss of Contact Inhibition

Differences among People in Society

Malaria

Afflicts:

235 Million

per year

causes diseases like

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191

Inheriting Genes 6

Vincent Van Gogh, the famous artist, was believed to have been afflicted with porphyria

The porphyria gene is on a chromosome

Porphyria treatment of the future

Porphyria gene on chromosome separates into sperm and egg

(a) Pedigree of Family with Porphyria

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192 Unit 2: Is it all in the Genes?

the Case of the Vampire Diary Diary entry: February 13, 2013.

Last month, during our college semester abroad, we experienced something I will never tell another person. I can’t be sure, and maybe I am crazy, but I know it happened… and it changed my life forever.

It all began when we spent a month in Europe. My mother’s family came from what was once the Austro-Hungarian Empire. They immigrated to America many years ago and did not speak much about their lives in the old country. The town they came from, Sibiu, was now in Romania. Before the wars, the area was German and was called Hermannstadt; before 1918, it was in the province of Transylvania.

My friend and I rented a small car, a Yugo, and made our way to Sibiu from Vienna. The day we left was hectic, and the sun was very bright. I did not like the bright sun; it always made my skin ache. It was just the two

of us taking a weekend away from the rest of our class, which stayed back in Vienna. We were friends, in a way more like acquaintances. He was bored of the party scene in Vienna and wanted to immerse in the local culture. So he decided to accompany me to my ancestor’s home town. He sure got what he wanted.

It was a cold night when we got to Romania, with clouds quickly moving overhead, making the moon appear ominous. Keep in mind, I wasn’t scared at all – I had no idea of what was yet to come. All of a sudden, the Yugo started to sputter. The car shut down and my friend yelled, “You dummy, you forget to add gas to this thing!” I was embarrassed and really felt bad about letting him down. I knew it was the sunlight that confused me when I picked up the rented Yugo.

We were tired, and there were no houses along the road. “At least it isn’t snowing,” I said meekly to try and break the cold mood between my friend and me. There was no response as we walked through the fields. There was also no road – it had ended at an open field with no sign of civilization. In the darkness, over on a hill in the distance, we spotted an old house. As we came closer, it was more like a hut, with clapboard walls and a rundown porch. I told my friend, “Let’s keep on going . . . Sibiu couldn’t be too far off.” I knew this was a lie but I had a bad feeling about the place. There was no response, and I knew my friend was bent on going to the house for gas.

ChECk In

From reading this chapter, students will be able to:

• Explain how inheritance of genes affects our health and society. • Trace the discovery of laws governing heredity. • Discuss Mendel’s experiments and the principles of genetics he derived from those experiments,

using and explaining terms such as dominant, recessive, Punnett square, and codominance. • Describe the stages of meiosis, its products and its role in fertilization. • Explain and give examples of single-gene characteristics in humans. • Enumerate and explain non-Mendelian patterns of inheritance, explaining how a pedigree can be

used to trace gene flow in families. • Use population genetics principles to trace gene flow in populations. • List and describe the branches of gene technology, evaluating its products’ impacts on human society.

(b) A Town in Transylvania. From Biological Perspectives, 3rd ed by BSCS

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Chapter 6: Inheriting Genes 193

We knocked on the door, with enough force to make it heard. After a long time with no answer, we started away. As we were leaving, an old lady opened the door. “Come in out of the cold. You must be Americans.” I told the lady that my family had come from this area a long time ago. The lady was the last of the Germans left in Transylvania. “You are one of us then!” she exclaimed. She came very close to my face, looking deep into my eyes. She remarked inappropriately, “You look like my father did when he was young.”

As we sat in her parlor we explained that we needed just a bit of gas to get us to the next town. The room was creepy, but the lady was very agreeable. “I’ll get my brother, Herbie, to fetch some gas.”After she left the room, we waited and waited, but no brother. Then, my friend felt something behind the couch – it was a man lying on the ground! “I see you have met Herbie,” said the old lady. “He’s been drinking and needs his bed. Would you help him up?”

This was getting to be too much, but each of us grabbed a limb to carry him. At that moment in time, we froze, looked at each other, and knew something we dared not say – this man was dead. His flesh was cold, and his skin bloated. Herbie’s skin was scarred, teeth were fangs, and his face appeared almost wolf-like. He looked just like a vampire. My friend and I looked at each other but said not a word.

We brought Herbie up to his bed and laid him down for one last rest. It was then that he sat up, looked at us and thanked us. He looked at me and said, “You look like my father!” I ran out of the house as fast as I could, maybe 15 miles to the town of Sibiu.

I now know that my family was from vampires; maybe their father was my grand- father or maybe I inherited their vampire ways, somehow. But I knew one thing – I am a vampire too.

ChECk Up sECtIon

In the story, Herbie has a blood disorder called porphyria. It is an inherited disease, occurring in about 1 in every 25,000 people. Enzymes that produce parts of his red blood cells, called hemes (which carry oxygen) are not formed properly. More specifically, heme groups, or substances that store oxygen in blood cells, are not formed correctly in porphyria. Without these enzymes, porphyrins (parts of hemes in red blood cells) build up, causing lesions in the body.

Symptoms include sensitivity to light (photosensitivity); craving for blood (due to a lack of heme groups); receding and bloody gums making teeth look like fangs; scabs and lesions from sun; organ damage; and rampant growth of hair in body parts to appear wolf-like. Porphyria sufferers need blood transfusions to replace their deficient hemes. We cannot be sure if the college student who narrates the story has inherited porphyria. Its symptoms usually appear during late adolescence. However, it is possible to manifest later in life.

Study porphyria to determine its genetic and/or environmental causes in more detail. How might porphyria have contributed to the myth of vampires in our society? Do you think the narrator in the story had porphyria, based on your research of the disease?

Unraveling the Mystery of Inheritance Chapter 5 described the molecular players in gene transfer; in this chapter, we look at the processes underlying inheritance. We begin in a small garden monastery in the 1800s. Gregor Mendel (1822–1884), an Austrian monk who failed out of a science teaching major in college, discovered how we pass traits onto the next generation.

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194 Unit 2: Is it all in the Genes?

By the mid-1800s, it was generally accepted that ova and sperm both contribute genetic information to new offspring. Most biologists believed, at the time, that inheri- tance from parents occurred as a blending of characteristics. In this view, traits from both parents averaged together to produce new, unique offspring.

Seeking to discover if there were specific patterns in the inheritance process, Men- del devised a set of experiments using pea plants as his subject. Using pea plants to study inheritance was not original, but his approach to understanding how we inherit our traits was unique. The passing of characteristics from parent to offspring is known as heredity. Through his experiments, Mendel was able to successfully develop the basic principles of heredity.

Mendel’s experiment was successful for a few reasons:

1) The garden pea plant Mendel chose was commercially grown at the time, repro- duced quickly, and possessed traits easily measured by simple observation. The garden pea plant self-pollinated, meaning that egg and sperm from the same plant would unite. The pea plant’s sexual structures were enclosed by a petal capsule, preventing cross-pollination from other plants. Therefore, Mendel could control cross-breeding with select plants and not worry about accidental cross-pollination.

2) Mendel chose measurable variables to study; those that were clearly discernible. He selected seven pea plant traits to study because they were clearly one of two alternatives. These seven traits included shape of seeds, color of seeds, shape of pods, color of pods, height of plant, color of flower, and position of flower. For example, plants had either round or wrinkled peas; and either yellow or green peas.

3) He used mathematics to measure and expose patterns in his results. Figure 6.1 shows the results of Mendel’s experiments. Note that the frequency of plant characteristics shows distinct proportions resulting from the crosses in each generation.

4) Mendel’s experiment was careful, logical, and sequential. His steps were metic- ulous and well thought out. Mendel credited any successful science experiment to certain attributes, stating in his original paper, “The value and utility of any experiment are determined by the fitness of the material to the purpose for which it is used.”

Heredity

The passing of characteristics from parent to offspring.

Figure 6.1 Gregor Mendel is the father of genetics and was an Austrian monk who discovered the laws governing patterns of inheritance.

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While Mendel’s findings were groundbreaking, they were unrecognized for 35 years. In 1865, Mendel reported his experiments and results in a paper presented to the Nat- ural Historical Society in Bruenn (now Brno, Czech Republic), the Austro-Hungarian Empire. Scientists in the audience dismissed his findings. Afterward, Mendel returned to his monastery, tending to his priestly duties; he was ignored and unappreciated by the scientific community. It was not until after his death, in 1900, that biologists began to build upon Mendel’s paper. Mendel’s work eventually was recognized and discredited the blending of traits perspective. Instead, his findings showed that traits were inherited as discrete units from each parent. Mendel thus began a scientific revolution in the field of genetics. Gregor Mendel is now recognized as the father of genetics for his work on pea plants. Let’s take a closer look at the laws of heredity that Mendel formulated so long ago.

Mendel’s laws law of Dominance Let’s revisit Mendel’s work: First, he chose to crossbreed certain plants. For example, Mendel noted that one variety of plant always produced yellow peas, while another pro- duced green. He took the male anther portion of a yellow pea plant and dusted the female stigma of a green. He called these original parents the F0 generation. When he crossed the two, all of the offspring were still yellow and none of them were green. This first cross Mendel called the F1 generation. Any trait that appeared in the F1 generation he called dominant for that characteristic. He surmised that any dominant trait covers up the alternative characteristic of an organism in the F1 generation. Next, Mendel crossed the organisms in the F1 generation in the same manner and their offspring were analyzed, the F2 generation. Mendel conducted what is now termed a monohybrid cross. This is a mating between two organisms, each having both characteristics for a particular trait – in this case both yellow and green. It is termed “mono-” because it looks at the inheritance of only one trait. Mendel surmised that although all of the plants of the F1 generation were yellow, they harbored a hidden green characteristic able to be given to offspring.

Mendel formed a hypothesis: the covered-up trait would reappear in the F2 gen- eration. Indeed, he predicted correctly that some offspring of all-yellow plants would be green. He was correct; the covered-up trait always reappeared in the F2 generation, bred from parents that did not exhibit the trait. The idea that a dominant trait covers up another is known as the law of dominance. He called the characteristic that is covered up the recessive trait. In his experiment, the yellow trait was dominant, and the green trait was recessive. The original parents, the F0 generation, he deduced, were each pure – the yellow parent had only dominant yellow characteristics and the green parent had only green characteristics – but that these characteristics would pass along in a predictable manner through each generation.

law of segregation When Mendel analyzed the F2 generation, he found that a certain proportion always appeared in his data. Note in Figure 6.2 that the F2 generation for all seven characteris- tics he chose had a roughly 3:1 ratio of dominant to recessive characteristics.

Because the appearance and disappearance of traits occurred in constant propor- tions, Mendel inferred that traits must be inherited as two separate, discrete units. We now call these units alleles. Alleles are alternate forms of the same trait. For example, if a pea has a yellow or green color possible, then either a yellow or green allele is

Dominant

The trait that covers up other forms of the characteristic.

Monohybrid cross

The mating between two organisms, each having both characteristics for a particular trait.

Law of dominance

The idea that a dominant trait covers up another.

Recessive

The trait that is covered up by a dominant trait.

Alleles

An alternative form of the same trait.

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196 Unit 2: Is it all in the Genes?

responsible for it. The hypothesis that there are two separate, discrete alleles that could be inherited separately is known as Mendel’s law of segregation.

We are now able to trace the movement of alleles from parent to offspring using a Punnett square. While this square was not actually used by Mendel, it derives from Mendel’s law of segregation. A Punnett square is a diagram based on the law of segre- gation that is used to predict the probability of inheritance of alleles between parent and offspring. Figure 6.3 uses a Punnett square to show how alleles are discretely passed on to a new generation. The mother’s alleles appear on one side of the box and the father’s on the other side. Each parent in the figure has two possible alleles based on their genetic make-up. Capitalized alleles are dominant and lower case alleles are recessive in the Punnett Square. Alleles from each parent have a 50:50 chance of segregating into an egg or sperm, eventually forming a new organism with a new genetic make-up.

• To recall from Chapter 5, an organism’s genetic make-up is known as its geno- type. The expression of that genotype is an organism’s phenotype. In other words, how an organism appears is its phenotype; and what comprises inside an organ- ism’s genes is its genotype.

The Punnett square gives the probability of producing an organism with a particular genotype within each box. Each box of the Punnett square represents a 25% chance that an organism’s genotype will appear in the offspring generation. Figure 6.4 shows the process of allele transfer between parents to offspring in porphyria. In our story, acute intermittent porphyria (AIP) is a dominant trait, meaning that if a person has one allele for it then he or she will have the disease.

law of Independent assortment In another set of experiments, called a dihybrid cross, Mendel mated plants tracing two different traits – pea shape and color. In a dihybrid cross both parents possess dominant and recessive characteristics for a particular trait. It traces the inheritance of two separate traits at the same time. The term “di-” is used because it looks at the

Law of segregation

The hypothesis that states that there are two separate, discrete alleles that could be inherited separately.

Porphyria

An inherited disease which is characterized by abnormal metabolism of the blood hemoglobin.

P generation

Long × Short

Purple × White Flowers

Axial × Terminal flowers

Green × yellow pods

Smooth × constricted pods

Yellow × Green seeds

Round × Wrinkled seeds

Total

All Long 787 Long, 277 Short 2.84:1

2.82:1

2.95:1

2.96:1

2.98:1

3.01:1

3.15:1

3.14:1

705 Purple, 224 White

651 Axial, 207 Terminal

428 Green,152 Yellow

882 Smooth, 299 Constricted

6,022 Yellow, 2,001 Green

5,474 Round, 1,850 Wrinkled

14,949 Dominant, 5,010 Recessive

All Purple

All Axial

All Green

All Smooth

All Yellow

All Round

All Dominant

F1 generation F2 generation Ratio

Figure 6.2 Results of Mendel’s experiments with pea plants F1 and F2 generations.

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Figure 6.3 Principle of segregation. Alleles separate into opposite ends of the cell. Alleles move independently of each other, according to Mendel’s laws, with equal chances of being transmitted to offspring. Note that any one of the four gametes produced by parents in the figure could be transmitted to the offspring generation. The Punnett square shows the relative probability for each gamete to give rise to its genotype.

F1 generation

F2 generation

9/16 Yellow and round (1 YYRR, 2 YyRR, 4 YyRr, 2 YYRr)

3/16 Green and round (1 yyRR, 2 yyRr)

3/16 Yellow and wrinkled (1 YYrr, 2 Yyrr)

1/16 Green and wrinkled (1 yyrr)

All yellow and round (YyRr)

Yellow and round YYRR

Green and wrinkled

yyrr

YYRR x yyrr YyRr x YyRr

YYRR YYRr YyRR YyRr

YR Yr yR yr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr

yr Yr

yr Yyrr yyRr yyrr

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Figure 6.4 a. A Punnett square for porphyria, a cross is shown between a parent with a dominant gene for porphyria and a normal parent for the F1 generation. The Punnett square shows that 50% of offspring will exhibit the disease. b. The disease porphyria is the likely basis for the legend of vampires.

(a) (b)

P p

P P

p p

p pP P

Parent with Porphyria

50% of offspring have the disease

normal parent

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198 Unit 2: Is it all in the Genes?

inheritance of two traits. One organism had yellow, smooth peas while the other had green, wrinkled peas. As shown in Figure 6.5, yellow and smooth are dominant traits, while green and round are recessive. When he analyzed the offspring of these crosses (the F1 generation), Mendel determined that traits were not inherited together. Instead they independently assorted as they were passed from one generation to the next. Wrin- kled and round were found alongside smooth and green. All of the possible types of pea plants showed up in the F1 generation of this dihybrid cross. In fact, these organisms also showed a pattern of proportions in a 9:3:3:1 ratio, with the dominant traits occur- ring most frequently (Figure 6.5). All new combinations of traits appeared in the next generation, each inherited separately from each other. The idea that each pair of alleles is sorted independently when sperm and egg are formed is known as Mendel’s law of independent assortment.

Two factors are inherited separately, one from a mother and one from a father. Thus, once together, they occur as either an identical pair or as a pair with different compo- nents. When a pair of alleles is the same, they are called homozygous. When both are dominant forms, they are homozygous dominant. When both are recessive, they are homozygous recessive. When alleles in a pair are different from each other, they are called heterozygous or hybrids for that trait.

In the porphyria case seen in our story, the disease is held on a dominant allele. Thus, if a person possesses an allele for porphyria, whether homozygous dominant or heterozygous, he or she will get the disease (see Figure 6.4). Only a homozygous reces- sive individual does not exhibit the disease. If P = the allele for porphyria and p = the allele for the normal condition, then an individual with PP, homozygous dominant or Pp, heterozygous will have the porphyria trait. Only a homozygous recessive, pp will have a normal blood condition.

Law of independent assortment

The idea which tells that each pair of alleles is sorted independently when sperm and egg are formed.

Homozygous

The condition in which a pair of alleles is the same.

Heterozygous

The condition in which alleles a pair are different from each other.

seed shape

seed color

seed- coat color

pod shape

pod color

flower position

stem length

round yellow colored inflated green on sides long

wrinkled green white constricted yellow at end short

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Figure 6.5 a. traits of pea plants, with the top row dominant and the bottom row recessive; b. Punnett square for a dihybrid cross for pea color and shape in pea plants. A cross is shown between two parents with both traits. The Punnett square below shows a 9:3:3:1 ratio in offspring characteristics.

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Chapter 6: Inheriting Genes 199

First-generation plants

When gametes are produced via meiosis all are YR.

When gametes are produced via meiosis all are yr.

a. b.

YR YR YR YR yr yr yr yr

When fertilization occurs, all possible combinations of gametes result in one kind of zygote.

YyRr YyRr YyRr YyRr

When these zygotes grow via meiosis into plants all body cells have YyRr genotype, therefore all produce yellow-round seeds.

These plants produce four kinds of gametes via meiosise.

YR Yr yR yr

yYrR

yYRR

YYrR

YYRR

YYRr

YyRR

YyRr

YyrR

YYrr

yYRr

yYrr

yyrR

yyRR

yyRr

Yyrr

yyrr

yr YR

When random fertilization occurs, 16 kinds of zygotes are produced

When these zygotes grow into plants, they produce third-generation plants with a 9:3:3:1 ratio of seed colors and shapes.

} } }

Yellow round Yellow wrinkled

Green round

Green wrinkled

= sperm

= egg

Second-generation plants

Third-generation plants

9 3 3 1

YR Yr yR yr

}

(b)

Figure 6.5 (continued)

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200 Unit 2: Is it all in the Genes?

testcross How do we determine the genotype of an organism?—, it is not always obvious from its appearance. Consider a green pea plant that inherits two green recessive alleles, one from each parent. It is green in its phenotype, indicating that it inherited two green alleles. If the plant had inherited one green and one yellow allele, it would have been yellow. When a yellow plant appears, it is more difficult to know its genotype without knowing its history. A yellow pea plant has a dominant allele, but does it have a recessive that is covered up, or is the other allele dominant as well?

Through using a testcross, the genotype of the yellow pea plant is explored. In a testcross, a known homozygous recessive organism, for example, a green pea plant is crossed with a yellow phenotype. The green pea plant, we know, has two green (reces- sive) alleles. But what is the yellow plant’s genotype? In this case, we do not whether the plant with yellow peas is homozygous dominant or heterozygous. In this testcross, a hidden recessive is most likely to be revealed. The homozygous recessive individual (the green plant) has the best chance of passing all its recessives to the next genera- tion. Figure 6.6 shows a testcross between a yellow pea plant and a green pea plant to determine whether or not green peas will result in their offspring. The testcross helps to determine the true genotype of the yellow pea plant.

The appearance of a recessive in the testcross’s progeny is the only definite proof that the unknown genotype was indeed a heterozygote. In other words, if one of its off- spring is green, then alleles coming from both parents must have been green. For the new offspring to have become a homozygous recessive, one recessive allele had to come from the yellow parent plant. However, if there is no individual with a recessive trait for pea color in the F1 generation, it may mean simply that the recessive allele may get expressed in another generation. Perhaps that recessive allele simply did not get passed along this time around. It is impossible to know for sure, but a testcross gives the best chance of being able to reveal the true genotype of an individual with a dominant phe- notype, such as the yellow plant.

This is also the reason recessives are so difficult to study and/or remove from a group. They are hidden, and only chance dictates whether or not an allele will become expressed. Many times recessive traits are deleterious, or cause harm to an organism hav- ing them. Many diseases are recessives and it may take several generations for a reces- sive disease to appear. It is hard to track recessives for this reason. For example, a family may be surprised that sickle-cell anemia is in their genetic history. Family members

Testcross

A known homozygous recessive organism is mated with a dominant organism.

Figure 6.6 Testcross for color on pea plants. A testcross always uses a homozygous recessive to attempt to reveal the recessive of its dominant mating partner. If even one of the offspring shows recessive characteristics, then the dominant partner harbors a recessive allele. In the example shown Y = yellow and y = green coloration in pea plants.

Y ?

Y y y ?

y ?Y y

unknown genotype

yy = if a green phenotype shows up, the unknown genotype contained a recessive allele ©

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may think it is not a risk because no one has had sickle-cell anemia, for as long as they can remember. However, it may have been hidden in the heterozygote condition for a period of time and was unexpressed. Deleterious recessives are a difficult but common thread in most groups. Some forms of porphyria are recessive, showing up many gen- erations after they are thought to be gone. Did our narrator in the story experience the reappearance of a long-silent recessive allele?

Meiosis: how sex Cells are Formed We have seen how traits are passed on from one generation to the next; now we will examine how organisms reproduce sexually. During Mendel’s time, it was accepted that parents transfer their hereditary information through a process called reproduction, to form a new organism. The central step in reproduction is fertilization, when a male and a female sex cell, both called gametes, unite. The female sex cell is the egg; the male, the sperm. Each contributes half the total genetic material that unites and recombines in the zygote. If the offspring receives genetic material from both parents, how is it that the offspring contains the same number of chromosomes as the parents? The answer is meiosis, which is a special form of cell division in which the newly produced daughter cells contain only half the number of chromosomes of the parent. This half-quantity is called the haploid or N condition, while the full complement of genes in all of our other cells, called somatic cells, is known as diploid or 2N. If a sex cell were not haploid, then the genes in the sex cells would double with each successive generation.

Every species has a set number of chromosomes. A mosquito has six chromosomes per cell; a sunflower, 34; a human, 46; a dog, 78; and a little goldfish, an impressive 94 chromosomes. In contrast, gametes of each of these species contain only half of these numbers: a mosquito gamete has 3 chromosomes, a sunflower, 17; a human, 23; a dog, 39; and a goldfish, 47 chromosomes.

In a diploid cell, each chromosome has a partner, much like a pair of shoes (see Fig- ure 6.7. The chromosome partners are known as a homologous pair. Homologs have one maternal and one paternal copy of a chromosome. Alleles on each homologous chromo- some code for the same trait. An allele for eye color, for example, on one chromosome codes for eye color alongside the allele on its homologous pair. Figure 6.7 shows the

Figure 6.7 Homologous genes, knows as alleles, occur at the same location and code for the same traits. From Biological Perspectives, 3rd ed by BSCS.

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Fertilization

Is the process in which male and female sex cells unite.

Zygote

A fertilized egg cell.

Haploid (N)

The half number of chromosomes of the parent.

Somatic cells

The full complement of genes in all of other cells.

Homologous

The chromosome partners in a diploid cell.

Diploid (2N)

The full complement of chromosomes in all body cells (except sex cells).

Meiosis

A special form of cell division in which the newly produced daughter cells contain only half the number of chromosomes of the parent.

Gametes

Reproductive cells (not given in bold in text).

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202 Unit 2: Is it all in the Genes?

alleles on a chromosome (in varied colors). Before meiosis and mitosis take place, homol- ogous chromosomes are duplicated. Thus, each replicated pair is composed of two sister chromosomes, identical to each other. Each set of duplicated homologous chromosomes contains four strands altogether: two original homologs and two duplicated strands.

Each homolog of the pair contributes one allele for a trait to its offspring. As shown in Figure 6.8, homologous chromosomes separate into four gametes during production of sex cells. Whether the individual homologue gets into a sperm or egg depends upon

Figure 6.8 Meiosis. Homologous chromosomes separate eventually into four sex cells (gametes). The doubling of genetic material takes place before the parent cell is able to divide. From Biological Perspectives, 3rd ed by BSCS.

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(a1)

chance, as described by Mendel. During meiosis, homologous chromosomes separate and move into one or the other of the gametes produced. They have an equal chance of entering a newly formed gamete because chance determines their entrance. Homologous chromosomes are inherited separately, as shown by Mendel’s law of segregation. Trace the movement of replicated chromosomes in Figure 6.8 to find the gametes’ destination.

The diploid number (2N) of chromosomes in a parent cell is divided equally into the sex cells during meiosis. Thus, the halving effect on the chromosome number occurs during gamete formation. The result is a set of haploid (N) sex cells. This halving effect counteracts fertilization, which unites genetic material from two sex cells into one somatic cell, the zygote or fertilized egg cell. The result is a unification of N + N = 2N.

As demonstrated in Figure 6.9, meiosis follows a series of stages similar to those seen in mitosis. Indeed, the names are also the same for the phases in both mitosis and meiosis. There are a few differences:

1) In meiosis, there are two sets of the same series of stages, meiosis I and meiosis II; but only one series in mitosis. This results in two cell divisions in meiosis and only one cell division in mitosis.

2) In meiosis, four new daughter cells are produced as a result of the two divisions, while only two are produced by mitosis.

3) Each daughter cell contains only the haploid number of chromosomes in meio- sis, but daughters in mitosis contain the diploid.

4) Gametes contain a variety of genetic possibilities, in part because homologous chromosomes separate into one or another of the sex cells, forming innumerable combinations.

Figure 6.9 a. Phases of meiosis. There are two stages of meiotic cell division, I and II. The end result of meiosis is the production of four haploid gametes (sex cells). Meiosis occurs in eight stages with descriptions of each stage given in the figure. b. Mitosis occurs in one division and results in two identical cells.

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204 Unit 2: Is it all in the Genes?

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Figure 6.9 (continued)

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Chapter 6: Inheriting Genes 205

the phases of Meiosis In a period before meiosis, the interphase carries out functions similar to those during the interphase before mitosis: cells grow in size, organelles duplicate and grow, and genetic material doubles in the nucleus. When genetic material doubles during inter- phase, two pairs of homologous chromosomes are formed. The purpose of meiosis is to produce daughter cells capable of fertilization. To do this, fertilization requires two haploid cells with haploid genetic material to unite.

During the first series of stages of meiosis, called meiosis I, homologous chromosomes separate (refer Figure 6.9 to see each stage). Just like the first stage of mitosis, when a cell begins meiosis, nuclear material condenses, transitioning from chromatin to chromosomes, its nuclear envelope disappears, and chromosomes attach to a spindle fiber. Unlike mito- sis, the first stage of meiosis I, called prophase I, homologous chromosomes in proximity to each other exchange genetic material through a process called crossing over. In cross- ing over, segments of one chromosome swap with segments of another pair. Crossing over enhances the genetic combinations possible in gametes, as shown in Figures 6.8 and 6.10. Areas that are crossed over randomly swap genetic material, leaving each homolog with a unique set of DNA.

In metaphase I, the homologous chromosomes line up as pairs, which later separate and move to opposite poles during anaphase I. Spindle fibers pull the pair of dupli- cated homologs into the center. In the next phase, anaphase I, homologous chromosomes

Table 6.1 Comparison of meiosis and mitosis

Courtesy Peter Daempfle.

Meiosis Mitosis

Number of cells Four new cells produced Two new cells produced

Number of divisions Two cell divisions One cell division

Genetics of cells Haploid cells made Diploid cells made

Compared to parents and each other Different (variability) Identical

Figure 6.9 (continued)

Interphase

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

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Metaphase I

The stage of mitosis and meiosis that follows the prophase stage and precedes the anaphase stage (not given in bold in text).

Crossing over

The exchange of genes between chromosomes.

Prophase I

Also called the first stage of meiosis I, in which homologous chromosomes in proximity to each other exchange genetic material through a process called crossing over.

Meiosis I

The process of cell division by which homologous chromosomes separate and new cells are haploid.

Anaphase I

The stage of cell division in meiosis in which homologous chromosomes separate.

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206 Unit 2: Is it all in the Genes?

separate to opposite ends of the cell. They are pulled apart in a random manner. A pater- nal homolog may be pulled onto one side while a maternal homolog may be pulled onto another side. At this point, the developing cells are haploid – with half the number of a complete set of chromosomes. With 23 sister chromosomes pairs, there are 2n possible new combinations. Thus, with 23 pairs of chromosomes in humans, there are 223 new possible genetic combinations in each newly formed gamete: 2 × 2 × 2 × 2 . . . 23 times! The genetic variation produced by random assortment is enormous.

Mendel hypothesized this random segregation of chromosomes, long before an understanding of the phases of meiosis. Thus, three sources of genetic variation among organisms are seen: 1) meiotic segregation of chromosomes; 2) random mutations in genes as discussed in Chapter 5; and 3) crossing over, as discussed earlier in this section. The processes of obtaining genetic variation are shown in Figure 6.10.

Figure 6.10 Genetic variation is introduced in species, especially during meiosis. a. Crossing over. b. Mutation. Recombination and independent assortment during seg- regation of alleles. All of these mechanisms add genetic diversity to cells and organisms. From Biological Perspectives, 3rd ed by BSCS.

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After this, telophase I and cytokinesis reform the nuclear envelop, with two new daughter cells containing their own nucleus. These new cells are haploid or N, contain- ing only half the original number of chromosomes. When homologs are pulled apart during meiosis I, sister chromosomes are placed in daughter cells. The genetic compo- sition of sister chromosomes is identical for the two. Thus, for these daughter cells, it is like getting two left shoes instead of a right and left. The two daughter cells of meiosis I are haploid, but contain a double set of half of the chromosomes.

Separating of sister chromosomes occurs during the next series of stages of meiosis, called meiosis II. A short period separates meiosis I and II in a brief interphase. In this time, there is no new duplication of genetic material and quickly cell division resumes into prophase II. Chromatids reorganize, coiling tightly once again as chromosomes, in preparation for the pulling apart process. During metaphase II, chromosomes line up singly and then the two sister chromatids (which are identical In anaphase II, identi- cal chromosomes separate, pulled apart by spindle fibers to opposite poles. In the last phase, telophase I, nuclei reform and chromosomes become tightly coiled once again. The physical separation of cytoplasm takes place during cytokinesis, as it pinches off to become two new cells. The end result of telophase II and cytokinesis, in which two new nuclei and cells form, is a total of four new haploid or N daughter cells.

As a result of meiosis, each human gamete contains only a haploid, 23 single strands of chromosomes, much like having 23 “left” shoes. It is fertilization by another gamete, containing 23 “right” shoes, that gives new life with a full diploid set of 23 pairs of chro- mosomes. Figure 6.11 shows chromosomes during meiosis represented as shoes.

Male and Female Gametes In animals, male meiosis produces four new sperm; and in females, one egg and three polar bodies form. All four gametes are haploid as products of male and female meiosis. Their nuclear material is evenly divided in both sexes. However, cytoplasm is unevenly divided in females. During a female’s telophase I, most of the cytoplasm is retained in one daughter cell, leaving the other three with very little cytoplasm.

Telophase I

The stage resulting in the forming of a set of new cells.

Meiosis II

The stage in which sister chromosomes are separated.

Metaphase II

The stage in which chromosomes line up singly and then the two sister chromatids separate and move to opposite poles of the cell.

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Figure 6.11 Chromosome separations of “shoes.”

Telomere

Nucleus

Cell

Chromosome

Prophase II

The first stage of meiosis II.

Cytokinesis

The division of cell cytoplasm following mitosis or meiosis.

Telophase II

The last stage in the second meiotic division of meiosis.

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208 Unit 2: Is it all in the Genes?

• Although note that not all animals reproduce sexually. For example, in ants, bees and wasps, a virgin birth (parthenogenesis) takes place to produce males, which will be discussed in Chapter 20.

During the next meiotic division, another unequal partition of cytoplasm happens. In most female animals, the result is a set of three small sex cells and one large sex cell. The daughter obtaining most of the cytoplasm becomes the female egg, while the others become polar bodies. Polar bodies generally disintegrate quickly and are not viable for fertilization. In human males, four gametes are made per meiotic division (Figure 6.12). However, many divisions occur simultaneously, continuously producing large numbers of gametes. The average ejaculation contains about 225 million sperm. In females, there is generally only one egg in a cycle. A great deal of energy is placed into egg pro- duction, but sperm are made en masse.

In most plant and animal species, the female gamete contains most of the cytoplasm. Can you deduce why? The egg will provide most of the resources, both nutrients and organelles, for a developing zygote. Once fertilized, an egg has the full complement of genetic material from unification with a sperm. Its cytoplasm provides an excellent

Figure 6.12 Gamete development. a. Females who carry out oogenesis (egg formation) and b. males, who carry out spermatogenesis (sperm formation). Both result in the pro- duction of four haploid gametes, but males produce four sperm (in the tubules of the testes) and females produce one viable egg with three polar bodies (within ovaries).

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Oogenesis

Mitosis

Meiosis I

Meiosis II

Primary egg arrested in prophase I

Primary egg

Secondary egg

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(completed only if fertilized)

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nutrient resource for the new organism’s survival. Also, by having instant organelles on hand for growth and development, the new embryo has an advantage. This is why mitochondria and chloroplasts are so beneficial to simply inherit, in accordance with the endosymbiotic theory – instant energy and food production for a new organism.

sex: a Cost–Benefit analysis Why sex? It has its advantages and its disadvantages for a species. Asexual reproduc- tion is more efficient and requires less cell machinery. Prokaryotes reproduce by binary fission, simply splitting in half to form two new organisms. The main disadvantage of asexual reproduction is limited genetic variation. Asexual reproduction perpetuates the genotypes of its parents, changing very little from generation to generation. Sexual reproduction instead, leads to many varieties of offspring, enabling some organisms to survive during changing conditions.

If, for example, a change in the environment should occur, as in the potato famine in Ireland in the 1800s, all asexually produced offspring will respond in the same manner. In Ireland, all of the potato plants at the time were grown asexually from the same origi- nal plant. The organisms produced, with the same genetic variety, were susceptible to the same fungal-like protist, causing them to decay and leading to famine. Genetic variation allows for differences in a group so that at least some will survive. Variety in potatoes in Ireland at the time would have saved over two million lives.

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Figure 6.12 (continued)

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210 Unit 2: Is it all in the Genes?

Sexual reproduction, on the other hand, allows new combinations of genes to form in offspring. Through crossing over, segregation, and mutation, many genetic combi- nations are possible. Of course, asexual reproduction allows for some variation due to random mutations in organisms’ gene sequences during replication, but overall it results in limited variety. Sexual reproduction gives a survival advantage in the process of evolution – it provides enough genetic variation among individuals to help them adapt, as a species, to environmental changes better than asexually reproducing organisms.

THe BeNeFITS oF Sex IS DeBATABLe

When observing the praying mantis’s mating ritual, in which a male has innate fear of its mate, one still wonders if it is all worthwhile. One hypothesis con- tends that a female bites his head off during copulation (the act of sex), in order to “ease his mind” and relax during sex (Figure 6.13). This allows more of his sperm to enter into her. She is not wasteful, and eats his whole body after sex in order to gain energy for her developing embryos. Sex can be very efficient in its quest to build a better species.

Figure 6.13 Praying mantis sex. Soon after copulation, she will bite off his head and consume his body for the energy to raise their young.

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Determining sex Upon closer inspection of the 23 pairs of chromosomes in humans, the final smallest pair are the sex chromosomes. The other 22 pairs are called autosomal chromosomes, which carry out a cells’ life functions. Human sex chromosomes are either X or Y chro- mosomes, and these determine the sex of an organism. If a human has both an X and Y chromosome, or is XY, it is male; and if it has two X chromosomes, or is XX, it is female. The sex chromosomes differ from each other in a number of ways: a Y chromo- some is much smaller than an X; a Y chromosome carries very little genetic information; and a person can survive without a Y chromosome. After all, human females carry only two X chromosomes. A karyotype, which shows a visual map of a set of chromosomes for an organism, is given in Figure 6.14. In some disorders, chromosomes fail to separate

Sex chromosome

The final smallest pair of the 23 pairs of chromosomes in humans.

Y chromosome

A sex chromosome that is found only in males (not given in bold in text).

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and an abnormal number of chromosomes are seen. For example, in disorders such as Down syndrome, an extra chromosome #21 is found after the failure of that chromo- some to separate during meiosis.

In ants, bees, and wasps, in the order hymenoptera, queens produce haploid males and diploid females, making females more related genetically to each other than to males. In fact, they share 75% similarity in DNA, because females have all of the same genes in common from their fathers. The father is haploid and has only one set of the same chromosomes to give to all of his daughters. This phenomenon is known as haplodiploidy, in which some offspring are haploid and some are diploid. This is a basis for close relationships in ants, bees, and wasp societies: they share duties very closely within colonies. In fact, most females within a colony give up sex altogether, remaining sterile castes whose main purpose is to serve the queen master (Figure 6.15). Many plants and all earthworms have both male and female parts; they produce both male and female gametes. Sometimes simple temperature determines sex, as in turtles, lizards, and reptiles. In turtles, cooler temperature eggs become males, while warmer temperatures elicit females.

Figure 6.14 Human karyotype.

Human karyotype

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19 20 21 22 Y X

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Figure 6.15 Worker ants helping their queen. Loyalty is strong for a queen who controls all aspects of ant society. In this image, worker ants move their queen’s eggs, serving both their queen and their future sisters who will hatch from those eggs.

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Mendelian traits: single Gene Characteristics Mendel did not yet know about molecular structures and the chemical idea of the gene discussed in chapter 5, but his explanation for their transmission was remarkably accu- rate for many traits: that there is pattern to their heredity and that they are inherited as discrete units. Traits that are determined by instructions on a single gene are called Mendelian characteristics, or single-gene traits. There are more than 9,000 single-gene human traits that follow the principles of Mendelian genetics. These are either-or char- acteristics: an organism has either one type or the other.

Mendelian characteristic (single-gene trait)

Traits that are determined by instructions on a single gene.

Sex IS NoT SexuAL PReFeReNCe

Most research supports a strong genetic basis of sexuality. Behavioral genetics is the research specialty that studies the genetic basis of behavior, including sexual preferences. Sexual drive and desire vary across a continuum in most animal societies from asexual (no sex) to hypersexual (excessive sex). It is not a simple like or dislike of certain attributes in the opposite sex. Studies of monozygotic (identical) twins show high contributions of genetic influences for sexual preferences.

Biological bases for sexuality lie in two factors in animals: 1) activity of the medial preoptic area of the brain (MPOA) and 2) DRD4 dopamine receptor gene. Dopamine is a neurotransmitter found in the brain. Neurotransmitters are chemicals that affect different parts of the brain. In humans and rats, for example, the greater the activity in the MPOA area and the greater the number of DRD4 receptors, the higher the sexuality rates in humans and rats.

A range in sexual drives and behaviors makes sense evolutionarily. Hypersexuality, or having many sex partners, may appear favorable for enhancing one’s reproductive success (more offspring with more partners), but this is not so—quality also counts. Consider that after fertilization, in many animals a seminal plug forms after a male ejaculates. If another partner enters, the plug is dislodged and this next partner is also able to produce a viable offspring. In promiscuity, the final partner is equally likely to father the child as compared with the first partner. Usually the last partner in is weaker, older, and has poorer quality genes than the first. In animal systems, hypersexuality is therefore selected against, with many partners leading to weaker offspring. Experimental evidence shows that hyper- sexual behavior in rats leads to decreased reproductive success for the female.

At the other extreme, asexuality, which is a lack of sex drive, is observed in about 1% of humans. Why do such genes persist? One obvious answer is that a lack of sexual attraction does not mean lack of sexual behavior.

On the other side, one would also expect homosexuality to be selected against as it does not lead to new offspring. Another hypothesis as to why “gay” genes remain in our gene pool is based on kin selection. Kin selection is the theory that evolution favors helping between family members or kin to augment the transmission of their related genes. People who do not have their own children are more likely to help their nephews and nieces (kin), who are 25% identical to them. This behavior perpetuates their own genes more than not having any children. Thus, there is strong evidence for a genetic basis of sexual preference and helping behaviors.

Neurotransmitter

Are chemicals that affect different parts of the brain.

Hypersexuality

The condition in which one has many sex partners.

Asexuality

The lack of sex drive.

Kin selection

The theory that evolution favors helping between family members or kin to augment the transmission of their related genes.

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Figure 6.16 Examples of single-gene traits. A variety of characteristics are controlled by a single gene pair. Tongue rolling, for example, is dominant over not being able to roll one’s tongue and attached ear lobes are recessive. What Mendelian characteristics do you have? a. Colin Farrel, shown here with his sister, has a wid- ow’s peak. b. This father and son both exhibit the tongue rolling ability. c. This man’s ear lobes are attached.

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Figure 6.17 Examples of traits in three patterns of inheritance: autosomal dominant, autosomal recessive, and sex-linked traits. Each method of inheritance depends upon the expression of genes. Pedigrees for each pattern of inheritance give affected and normal individuals in each generation. From Biological Perspectives, 3rd ed by BSCS.

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Consider being able to roll your tongue or not roll your tongue; having a widow’s peak or not having a widow’s peak; and having albinism or not having albinism. Each is determined by whether one has dominant or recessive sets of alleles. A person who has a widow’s peak has a dominant allele dictating that the characteristic will show up. Figure 6.16 illustrates a few single-gene traits.

There are three possible patterns of inheritance of single-gene traits leading to an organism’s outward appearance: 1) autosomal dominant, in which the dominant allele gets expressed, 2) autosomal recessive, in which both recessive alleles are present for a person to get the recessive trait, and 3) sex-linked, in which the X chromosome deter- mines the characteristic (Figure 6.17). Each pattern follows Mendel’s rules, expressing the dominant allele in the phenotype. Examples of traits for each pattern are given in Figure 6.17.

Autosomal Dominant. Diseases that are autosomal dominants are expressed when even one allele is contained within a genotype. For example, in Huntington’s disease, a degenerative and progressive muscular illness, the trait is inherited as a dominant allele. If a person receives the autosomal dominant Huntington gene, she or he will develop its related disease. Symptoms usually develop after an age of 30 years, well after she or he could pass it onto children.

Singer Woody Guthrie, who sang “This land is Your Land,” died from the disease at an age of 55 years, 13 years after symptoms appeared. He was the father of singer

Sex-linked

One of the three possible patterns of inheritance of single- gene traits in which the X chromosome determines the characteristic.

x chromosome

A sex chromosome that is found twice in females and singly in males (not given in bold in text).

Autosomal dominant

The patterns of inheritance of single- gene traits in which the dominant allele gets expressed.

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214 Unit 2: Is it all in the Genes?

Arlo Guthrie, who did not inherit the disease from his father. Arlo had a 50:50 chance of getting Huntington’s disease. Its origin is thought to have arisen from a small town in Venezuela. About 30,000 Americans suffer from the disorder today.

Autosomal Recessive. Most diseases are carried on recessive alleles. Recessive alleles stay hidden within a genotype without being expressed for longer periods of time than autosomal dominants. As discussed earlier in this chapter, a person may harbor a recessive allele without knowing it is present; the dominant allele covers its effects within the genotype. Thus, deleterious recessives persist in groups.

For an autosomal recessive trait to be expressed, an individual must inherit one recessive allele from each parent. Thus, two unaffected individuals have a 25% chance of having an affected child. In Xeroderma pigmentosum, lack of DNA repair enzymes due to recessive alleles leads to skin lesions and skin cancers

While certain forms of porphyria as described in our story are inherited in an auto- somal dominant pattern (AIP), other forms occur through an autosomal recessive pattern (congenital porphyria). In both forms, those affected lack enzymes to produce heme groups in red blood cells. Because oxygen is carried throughout the body by heme groups lack of heme causes damage to body systems. (Human systems will be discussed in later chapters.) Both dominant and recessive porphyria are difficult to treat because insufficient blood causes irreversible damage to vital organs. In January 2013, it was reported that the remains of the mad King George III of England were discovered. His mental health as a leader was in question throughout his reign. King George III likely suffered from porphyria. His mental deterioration and decline are chronicled in the 1994 film, The Madness of King George. Many of the royal families married kin; increasing chances for inheriting harmful genes, such as porphyria.

Sex-Linked. Sometimes males have a greater chance of inheriting a trait than females. This occurs in sex-linked traits, in which a trait is determined by a gene located on a sex chromosome, making inheritance patterns different between males and females. In sex-linked traits, such as in color-blindness, often the disease-causing allele is recessive. Most genes are found only on the X chromosome, so it determines the expression of a trait.

If a female has one gene for color blindness, for example, she will not become color blind if she has another dominant, normal gene on her other X chromosome. The dom- inant allele masks the recessive allele causing color blindness. Alternatively, the same situation in a male would result in color blindness. A male does not have two X chro- mosomes to hide the one troublesome, recessive gene. Because a male has a Y chromo- some, which has very little genetic information, it does not hide the effects of the normal dominant allele. Sex-linked traits are more common in males than in females because of this pattern (see Figure 6.18). Females have greater opportunity to hide alleles with genes from their other X chromosome.

not so Mendelian Genetics Most traits do not act as Mendel predicted. How do we explain why there is not simply one or two possible skin colors? If all traits were Mendelian, all organisms of a species would have either one phenotype or another, with no variations in between. Obviously, this is not the case for most organisms’ characteristics. Other inheritance patterns pro- duce the phenotypes most common to us: skin color, IQ, blood types, height, weight, and sexual preference to name a few. While Mendel had great insights into his data, most of our genetic expression is more complex than the seven pea plant traits he chose to study.

Autosomal recessive

The patterns of inheritance of single- gene traits in which both recessive alleles are present for a person to get the recessive trait.

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Incomplete Dominance Incomplete dominance results from two different alleles contributing to gene expres- sion. Snapdragon plants, for example, occur in red and white varieties, but may produce pink flowers when mated together. A cross between a white and red Snapdragon plant is shown in the Punnett square in Figure 6.18. The red and white alleles are equally expressed in snapdragons, resulting in a pink color.

Multiple alleles Some traits are controlled by several genes, each expressing a particular phenotype. These traits are examples of multiple allelism. Individuals still carry only two of the multiple alleles at any one time, one from a father and one from a mother. However, the traits are all expressed within a population. In human blood groups, there are three alleles controlling blood types: allele A, allele B, and allele O. Alleles A and B are codominant, or share dominance with each other, and allele O is recessive. When allele A or B are present with O, as in AO or BO, the result is a blood type of A or B, respec- tively. When A and B are inherited together, a blood type AB results, and when allele O is homozygous with OO as the genotype, the result is blood type O.

Alleles code for antigens, or special proteins on plasma membranes of red blood cells: allele A codes for an A antigen, allele B codes for a B antigen, and allele O codes for no antigen. Antibodies are chemicals made by the immune system that initiate an attack on foreign bodies. When blood types with foreign antigens mix, antibodies are made against antigens found on red blood cells.

Incomplete dominance

A genetic situation in which one allele does not completely dominate another allele.

Multiple alleles

A series of three or more alternative forms of a gene, out of which only two can exist in a normal, diploid individual.

Figure 6.18 a. Sex-linked traits. Inherited on the X chromosomes, they are more likely to appear phenotypically in males. b. Snapdragons. Red and White Cross of F1 generation results in pink plants. 50% of the offspring exhibit incomplete dominance, showing a pink coloration. This phenotype was not seen in its parents. From Biological Perspectives, 3rd ed by BSCS.

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216 Unit 2: Is it all in the Genes?

To apply this, blood type O may be donated to any other blood group because it contains no antigens on its red blood cells for which to attack. Blood type O-is therefore called the universal donor. Blood type AB contains both A and B antigens on the red blood cells. Therefore, a person with blood type AB is able to receive all other blood types because they appear non-foreign to an AB immune system – all of the antigens are already on its red blood cells. Blood type AB+ is therefore called the universal recipient. Blood type A cannot donate to blood type B and vice versa. Blood type A has A antigens and makes antibodies for B (because B appears foreign to it). Blood type B has B anti- gens and makes antibodies for A (because A appears foreign to it).

Note that “+” and “−” have been used to describe blood types. Blood is classified as either positive “+” or negative “−” because of a surface protein marker on red blood cells, called the Rh factor. If blood contains an allele coding for the Rh marker, then its blood is considered positive. Type A+ blood contains at least one allele for the A antigen and one allele for the Rh factor. The Rh marker is another substance for immune cells to recognize and attack. Those with Rh positive blood types are able to receive Rh negative blood. Those with Rh negative blood, however, are not able to receive Rh positive blood. Rh positive blood contains the Rh marker, which would be recognized and rejected by immune cells of an Rh negative person. Figure 6.19 shows the four blood types along with their antigens and the red blood cells associated with each blood type.

polygenic Inheritance Most of an organism’s characteristics are polygenic traits, which are traits with patterns of inheritance determined by more than one gene and influenced by the environment. These include height, skin color, eye color, weight, hypertension, cancer, and heart dis- ease. Polygenic traits are said to be continuous, with many levels expressed along a bell- shaped curve. Figure 6.20 shows the curve for height in athletes as they have changed in the past century. Both exhibit a polygenic bell shape, but the average has increased con- siderably. What factors in society have changed to increase average height in our society?

Dominance or recessive expression is not so clear cut for polygenic traits. We are not either short or tall, strong or weak, a smart or a bad student or even a brown or blue eye color. There are many variations in between these extremes. Most individuals cluster around an average with very few found at the extremes.

universal donor

A person of blood type O who may donate blood to any other blood group because the blood group contains no antigens on its red blood cells.

Polygenic traits

Are traits with patterns of inheritance determined by more than one gene and influenced by the environment.

Figure 6.19 Codominance and multiple alleles. a. There are four discrete blood types in humans: A, B, AB, and O. Three different alleles determine blood type. Blood type is expressed as codominance with alleles sharing a phenotypic expression. b. Genetics of the human ABO blood groups. From Biological Perspectives, 3rd ed by BSCS.

Antigen A

Anti-B Antibody

(a) (b)

Type A

Red Blood Cells

Plasma

Anti-B Antibody Anti-A Antibody

Type B

Neither Anti-A nor Anti-B Antibodies

Type AB

Anti-A and Anti-B Antibodies

Type O

Antigen B Antigens A and B Neither Antigen A nor B

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A person of blood type AB who may receive blood from any other blood group because the blood group contains all antigens on its red blood cells.

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Polygenic traits are influenced by the environment because genes alone do not explain the variation in phenotypes. They are called multifactorial traits because they have many factors that affect their expression. Environment interacts with genes to form a phenotype. Obesity, a polygenic trait, was studied to determine the effects of genes and the environment on its expression in humans and mice. Identical twins, which have the exact same geno- types because they arise from the same fertilized egg, were studied. Twin studies often mea- sure how much a polygenic trait is due to genetics. Obesity had a concordance rate of 70%, meaning that 70% of the time obesity is found in both twins, regardless of what they ate.

The mouse Ob gene encodes for a weight-controlling hormone, leptin, produced in fat cells. Figure 6.21 shows two mice, one overweight, with a mutated Ob gene, and one normal weight, with a normal Ob gene. The human gene for leptin is on chromosome #7 and its mutation increases the risk for developing obesity. However, a mutation of the leptin gene is not the only contributor to obesity. Obesity is a complex disorder, involving the interaction of several genes with the environment. Indeed, scientists have detected genes for obesity in humans on chromosomes: #2, #3, #5, #6, #7, #10, #11, #17, and #20. Research on this multifactorial condition continues.

Figure 6.20 People are often categorized by their height. The mean height of men today is 5'10", whereas in 1913 it was 5'8". The photo from 1913 shows a group of college students categorized by height. Note that the categories follow a bell-shaped curve, a characteristic of polygenic traits. What factors do you think contributed to the change in average height over the past century?

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Figure 6.21 (a) Normal vs. (b) chubby rat, the ob gene has its effects on weight in rats (normal rat on the left and obese rat on the right.)

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218 Unit 2: Is it all in the Genes?

In our story, porphyria symptoms emerge from genetic and environmental factors. While there is a genetic component, stress, smoking, alcohol, and sun exposure trigger symptoms of porphyria. It is also shown that garlic aggravates porphyria symptoms, possibly the root of the assertion that garlic keeps vampires away.

There are eight enzymes involved in heme biosynthesis. Each enzyme has genes that code for it. If any one of these genes is mutated, abnormal heme production results. Thus, the disease has genetic roots as well as environmental triggers. It is multifactorial because many (or multiple) factors affect its expression.

Some polygenic traits are due to gene–gene interactions with very little environmen- tal input. For example, eye color is influenced by about 16 different genes, with less than 1% of its phenotype due to the environment (Figure 6.22). You may have assumed that eye color is an either/or scenario, but in fact it is a polygenic trait, with a continuum of colors possible. Have you ever wondered how hazel or green eyes develop? It is a matter of pigments. The more genes inherited for pigmentation in the eye’s iris, the darker the coloration. If there are no alleles for pigment production in one’s genotype, eyes will be blue; if there is one or two genes, eye color will be green; if there are three or four alleles for pigment, coloration will be hazel, and more alleles for pigment give varying shades of brown.

pleiotropy When one gene affects more than one trait, this effect is called pleiotropy. Several spe- cies of farm birds – chickens, turkeys. – exhibit a “frizzle” mutation on one of their genes. The frizzle allele causes bird feathers to be stringy and weak, providing poor insulation. More seriously, the mutated frizzle allele affects the bird’s heart, kidneys, and thyroid and impairs its overall health. Pleiotropy is seen in many characteristics from phenlyketonuria (PKU) in humans, with effects on brain and skin functions, to multiple congenital deformities in rats. All of the associated features of the disorders are due to a single-gene effect on multiple traits.

tracing Gene Flow in Families: pedigree analysis Pedigrees are diagrams of genetic relationships among family members through dif- ferent generations; they are used to trace gene flow through a family (see Figure 6.23 as an example). They show patterns and help figure out whether one has a dominant or

Pleiotropy

The condition in which one gene affects more than one trait.

Figure 6.22 Eye color genotypes and phenotypes. Eye color is mostly written in our genes.

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recessive allele, based on one’s parents. The pedigree diagram uses circles to indicate females and squares for males. No shading indicates unaffected individuals, shaded are affected and half shaded are known carriers or heterozygotes. Horizontal lines between circles and squares show mating and vertical lines show descent. Several traits shown in Figure 6.23 indicate how genes are expressed through generations: pedigrees for hemophilia and a family with porphyria are given.

tracing Gene Flow in Groups: population Genetics How do genes move between villages, cities, and continents? If you compare groups that are separated geographically do you find different characteristics? Is there such a con- cept as “race,” genetically separating different groups of humans? These questions have answers in the branch of genetics called population genetics. A population is defined as a group of individuals able to breed with each other in a given area, producing fertile off- spring. The study of patterns of gene flow from one group to another and within groups is known as population genetics.

Among other things, population geneticists investigate how diseases are carried in a population of organisms. Mathematical calculations determine the frequency of alleles in a group. These numbers help determine trends in gene flow over time. Porphyria was found to be in high proportions in populations in the old Austro-Hungarian Empire’s province of Transylvania, where the myth of vampirism originated in our opening story. Further studies are being done to determine the exact origins. In the example of Hun- tington’s disease, however, population geneticists determined that the gene arose from one woman in a small town in Venezuela, according to records dating back to the 1700s. Scientists collected information from 90,000 people and developed pedigrees to chart gene flow. They tested blood samples to detect the disease and plotted its movement through the years. Though Huntington’s disease is inherited, a 2001 study indicated that roughly 10% of cases result from new, random mutations.

Understanding how genes move within a population can help explain why certain genes persist in that population, and this in turn enables us to better understand diseases

Pedigree

Are diagrams of genetic relationships among family members through different generations; they are used to trace gene flow through a family (not given in bold in text).

Population genetics

The study of patterns of gene flow from one group to another and within groups.

Figure 6.23 A pedigree shows the genetics of a family tree. a. Symbols are used to create the family tree. This pedigree shows the inheritance of hemophilia by the royal families of Europe. Hemophilia is a sex-linked trait. The Bettmann Archive. b. A pedigree of a family with congenital porphyria, a recessively linked trait. From Biological Perspectives, 3rd ed by BSCS

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and organism characteristics. For example, by mapping out where cystic fibrosis is located geographically, scientists determined its benefits to immunity against cholera.

It is difficult to determine the exact number of carriers in a population because car- riers exhibit a normal phenotype. However, scientists may use a mathematical formula to estimate the probability of occurrence of a recessive allele in a population. The Hardy– Weinberg quadratic equation for equilibrium shows the relative proportion of alleles in a population through counting the number of recessive individuals:

p2 + 2pq + q2 = 1

In the equation, p equals the proportion of dominant alleles in a population and q equals the proportion of recessive alleles within a population. Homozygous dominant organ- isms are given as p2 and homozygous recessive are given as q2. Heterozygotes or carriers are given as pq. Through counting the number of dominant individuals, which one is able to detect through observation, p is calculated. Then, q is solved for, and the rest of the equation’s letters are calculated using the quadratic equation. This is a quadratic equation set equal to one. It assumes that a population is not evolving or changing in its allele frequency. It assumes no immigration, emigration, natural selection, or mutations that alter normal gene frequency.

Obtaining data through use of the Hardy–Weinberg equation helps determine the risk of having a particular gene within one’s population, helps understand if a popula- tion, such as a stand of red maple trees, is undergoing a change in gene flow, and exam- ines how populations compare with each other based on genetic factors. For example, with respect to the alleles for sickle-cell anemia: African American populations with West African ancestry have a 12.5% prevalence of sickle-cell anemia, but West Afri- can populations have a 20–40% prevalence. This indicates that the populations have diverged in their overall genetic compositions.

Genealogy is the study of family history. It is related to population genetics, using pedigrees to investigate one’s family history. New tests are available that allow one to send in a blood or saliva sample and have it analyzed to trace genetic origins. For instance, tests identify over 400 different ethnics groups in Africa from which our genes may be compared to determine origins. Is this useful or does this further divide people based on the social construct of race?

INBReeDING: Too CLoSe FoR CoMFoRT oR A GooD STRATeGY?

Consanguinity, or sharing blood through mating with close relatives, such as broth- ers and sisters, has been shunned by most societies throughout history. The cul- tural taboo has a practical origin: inbreeding depression, or the loss of heterozygotes and at the same time, the acceleration in the number of recessive alleles in a pop- ulation that are often harmful. The Hardy–Weinberg equation shows the increase of both recessives and their related diseases in studies of inbreeding groups.

Individuals in the same family share many genes in common. The recessive genes that would otherwise be covered up by the dominant allele are more

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likely to become expressed when recessives occur more frequently. It is likely that pockets of porphyria existed in Medieval Europe, where intermarriage was somewhat common. Porphyria would have been more pronounced in such areas, where dominant normal alleles for heme formation were less prevalent. When close relatives mate, both are part of a lineage that has the potential of sharing more of the same harmful genes in common. Examples may include sickle-cell anemia, cystic fibrosis, or even cancer.

On the other hand, recent research shows that a certain amount of inbreeding can produce healthier children. In a study of Iceland’s family history lineage, marriage between third and fourth cousins produced the most numer- ous and healthiest children over the past 1,000 years. It is hypothesized that outbreeding, or mating with someone too different from one’s own genotype, may also lead to health problems in children. In fact, about 20% of marriages worldwide occur between first cousins. This practice is illegal in many of the United States.

Outbreeding too far also has negative consequences, though. One such example occurs for the Rh factor, cited earlier in the chapter. Rh is a set of protein markers on red blood cells that need to match between mother and child for a healthy baby to be born. If the mother is Rh negative, and the father is Rh positive, then the blood of the second fetus who is Rh positive (from the father) will be recognized as an invader by the maternal immune system. Presently, Rhogam is a treatment given to pregnant mothers to prevent mis- matched blood from causing a problem (Figure 6.24). Without modern tech- nology, however, such a match would be disfavored. Thus, there is an optimal level of inbreeding for reproductive success. However, third and fourth cousins have only about 1/256 to 1/512 genes in common with one another, so the chances of revealing recessive alleles is quite low.

Figure 6.24 Rhogam is used to treat Rh incompatibility between mother and fetal blood types.

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Gene technology: solving problems Using Genetics Biotechnology is the branch of science that uses biological knowledge and processes to produce goods and services for human use and financial profit. Its techniques manip- ulate genetic sequences in organisms to produce medical drugs and develop weather- and pest-resistant crops, to name a few examples. One significant sub-branch is gene technology, which modifies plants, bacteria, and animals to create products for society. First, the genome of a specific organism is modified by inserting a gene from another organism into the subject organism’s already existing DNA. The resulting organism is called a genetically modified organism (GMO) and it is classified as transgenic because it contains genes from another species. Inserted genes produce proteins, for which the inserted gene codes. Human proteins such as insulin, to help diabetics, human growth hormone or HGH, to help in dwarfism, and factor VIII to help hemophiliacs are pro- duced by these GMO organisms. Transgenic tobacco plants produce HGH, as shown in Figure 6.25.

Before gene technology, the available means of collecting these proteins had many drawbacks. HGH was collected from dead bodies and could cause disease when injected into patients, for example. Hemophiliacs, who suffer from life-threatening blood loss due to the lack of a blood clotting factor, were dependent on blood transfusions, which carry a risk of containing infected blood. Before AIDS was discovered in the early 1980s, many hemophiliacs were infected with HIV from blood transfusions. Gene technology changed their treatment options, leading to less risk. Hemophilia is now treated with genetically produced clotting factor VIII. Lessened risk from disease-causing agents is a great step forward for society due to gene technology.

GMOs are produced through genetic engineering, which is the manipulation of an organism’s genes in a way other than is natural. This manipulation is accom- plished through using a technique called recombinant DNA technology (Figure 6.26). Recombinant DNA technology is the process by which DNA is extracted from nuclei of organisms and treated with restriction enzymes. Restriction enzymes cut DNA at spe- cific sequences. A bacterial plasmid, which is a circular strand of DNA, is also cut with

Figure 6.25 Transgenic tobacco plants. These plants are being used to produce human growth hormone (HGH) to treat human growth disorders. A gene has been inserted into these plants to produce HGH.

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Gene technology

The technology that modifies plants, bacteria and animals to create products for society.

Genetically modified organism

Are organisms in which DNA is genetically altered via genetic engineering techniques.

Recombinant DNA technology

The process by which DNA is extracted from nuclei of organisms and treated with restriction enzymes.

Biotechnology

The branch of science that uses biological knowledge and procedures to produce goods and services for human use and financial profit.

Genetic engineering

The process in which an organism’s genes are manipulated in a way other than is natural.

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Figure 6.26 Genetic recombination techniques. They are steps used in producing a genetically modified, transgenic organism. Note that the restriction enzymes cut DNA at specific locations, allowing plasmid DNA to attach and become a “part” of the DNA of the newly created transgenic organism. In this figure, the clotting factor VIII gene is inserted into bacteria in order to produce factor VIII en masse for human use. The bacteria made by genetic recombination are genetically engineered “transgenic” organisms. From Biological Perspectives, 3rd ed by BSCS.

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the same restriction enzyme. Bacterial and human DNA fragments are mixed together, causing them to link with each other. The bacterial plasmid now contains the human gene that will be used for coding new proteins. The plasmid is then transferred into a new bacterial cell. This bacterial cell expresses the newly inserted gene to make the desired protein. It divides over and over, forming new cells that make the a product. The bacterium with its newly inserted gene is said to have been recombined.

In our story, Herbie might benefit if biotechnology treatment options were available for porphyria. To date, porphyria is treated with limited success, with symptoms and long-term problems plaguing its sufferers. An area of study that holds promise for more successful treatment of porphyria and other diseases is gene therapy. Gene therapy is the insertion of genes into an organism to treat its disease. In the past two decades, gene therapy has had mixed success. Future research may find a way to insert a gene into por- phyria patients such as Herbie, that blocks the mutated gene, which is unable to produce normal heme groups. Another advance for porphyria sufferers would be in the area of blood production. Presently, blood transfusions restore deficient heme in the blood of porphyria sufferers. Panhemin is also a drug used today to treat porphyria by limiting the liver’s production of porphyrins (Figure 6.27). Both treatments are derived from human blood and have risks of carrying infectious agents.

Gene therapy

The process in which genes are inserted into an organism to treat its disease.

Figure 6.27 Panhemin Vial.

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ARe PRoDuCTS oF BIoTeCHNoLoGY HeLPFuL oR HARMFuL To SoCIeTY?

Many products are made available through the use of biotechnology. Trans- genic crops, for example have greater resistance to herbicides, and viral and fungal diseases. They are modified to withstand cooler temperatures longer and grow faster with larger fruits and vegetables. Soybeans, corn, cottonseed, and canola crops have seen large increases in transgenic numbers in the past decade, as shown in Figure 6.28. Over 93% of all soybeans and cotton crops are genetically modified in the United States. Eighty six percent of all corn, a major staple for cattle and humans, is produced by GMO organisms. If these organisms were not permitted to contribute to our food supply, would we be able to sustain our need to produce food, as a world population?

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There have been big increases in farm production since the development of GMO foods. Crops are hardier and more productive, but it is a hotly debated area of study. The greater abundance of food means that fewer people go hungry. However, some GMO foods may also be linked to disease. A 2012 study in Europe shows that a corn variety, NK603 containing genes making it more resistant to the weed killer Roundup, was shown linked to cancer-caus- ing effects when fed to a group of mice. Owing to this “cancer corn,” some European nations are placing restrictions on transgenic products. Is this fear of NK603 corn justified?

The public has been consuming GMO products for over 15 years. No known ill effects have been confirmed by the scientific community in this time. What effects will be shown in 10, 20, or 30 years from now, is yet to be determined? Long-term results are not available because GMOs have not been around long enough.

Figure 6.28 Graph showing relative increase in transgenic crops over the past 20 years.

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2004 2005 2006 2007 2008 2009 2010 2011 2012

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the things We’ve handed Down: should We tamper With our Genes? HGH produced by gene manipulation for the past 25 years helps extreme cases of growth disorders. Before recombinant DNA techniques were available, HGH was extracted from the pituitary glands of cadavers and carried the risk of contaminating patients. HGH is now fast and easy to produce, without contamination risks, making it more commercially available.

This brings ethical and practical medical questions into play: Should a preteen male, predicted to grow to a height of about 5' 4", take the drug? What if it is against the doc- tor’s advice, which is based on the American Medical Associations guidelines to restrict the drug only for extreme cases? What is an extreme case? What are the side effects? These are difficult questions to answer.

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HGH has known side effects – from abnormal growth of joints to chronic pain, dis- figurement and even death. HGH is used more and more in society to help teens reach a desired height. Some people are very happy with the results and other are devastated. There is uncertainty in medical procedures and treatments and their risks and benefits should be weighed.

What are the social issues involved in being short? How many female readers would date a person taller than they are? I presume many would. How many would date a per- son shorter than they are? I am not sure. How many male readers would date a person shorter than they are? I presume most. How many male readers would date a person taller than they are? I am not sure.

The reader should weigh the pros and cons of using technologies that scientists have made available to them. It is difficult to judge one another’s decision without under- standing the social implications of the medical treatment.

Ethically, will another doctor help the patient if one doctor denies treatment? What is ethical for doctors to do if a patient desperately wants treatment to grow taller? These are just a few of the provocative questions about HGH that some young Americans face every day.

MYTH, FABLe, AND SuPeRSTITIoN HAve exPLAINeD MALADIeS INCoRReCTLY

In the story at the beginning of the chapter, the character ponders his iden- tity as a vampire. Throughout history, unexplainable illnesses have often been linked with folklore of the occult, witchcraft, and creepy creatures. Epidemics of the plague, consumption (tuberculosis), and the like often led to exhuming bodies and labeling one or more victims to some sort of myth or fable based upon fear. All of these illnesses had biological origins, as shown in our opening story, which shows the power of myth in shaping a person’s mental health and outlook—Was the narrator in the story a victim of his own superstitions?

summary Heredity is the study of inheritance of characteristics from parent to offspring. Predicted patterns of inheritance were discovered by Gregor Mendel in the 1800s. Mendel’s three laws describe inheritance of over 9,000 human traits. Inheritance of genetic informa- tion is more complex than Mendel hypothesized. Genes interact with each other, the environment, and sometimes share in their expression. Sexual reproduction results in a great deal of variation in populations. Meiosis, the forming of sex cells, produces unlim- ited genetic combinations within gametes. The flow of genes from one group to another is studied by population genetics. The numbers of different genotypes and phenotypes in a population are given by using the Hardy–Weinberg equation, with certain assump- tions accepted. Biotechnology’s important component, gene technology, has resulted in many products available for public use. Gene technology products are continually being developed. Their effects on society and science continue to be debated as well.

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summary: key points

• Heredity affects our physical characteristics, our environment and our future generations. • The discovery of inheritance by Gregor Mendel explains many of life’s characteristics. • Inheritance can be explained in future generations by probability using Punnett squares and in

populations using the Hardy–Weinberg equation. • The stages and products of meiosis explain how sexual reproduction leads to great genetic variation. • Many traits in organisms are non-Mendelian, explained by codominance, polygenic inheritance,

multiple alleles, and pleiotropy. • Pedigrees clarify gene flow within families. • Population genetics studies gene flow between and within populations. • Biotechnology has advances to provide products for human use, with debatable effects.

alleles anaphase I, II asexuality autosomal dominant autosomal recessive biotechnology crossing over cytokinesis diploid (2N) dominant fertilization gametes gene technology gene therapy genetic engineering genetically modified organism (GMO) haploid (N) heredity heterozygous homologous homozygous hypersexuality incomplete dominance kin selection law of dominance law of independent assortment

law of segregation meiosis, meiosis I, meiosis II Mendelian characteristic, single-gene trait metaphase I, II monohybrid cross multiple alleles neurotransmitter pedigree pleiotropy polygenic traits population genetics porphyria prophase I, II recessive recombinant DNA technology sex chromosome sex-linked somatic cells telophase I, II testcross universal donor universal recipient X chromosome Y chromosome zygote

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Multiple Choice Questions

1. Which is an inherited disorder? a. porphyria b. obesity c. Huntington’s disease d. all of the above

2. Which of Mendel’s laws was derived from the presence of 100% yellow phenotypes in the F1 generation? a. law of dominance b. law of independent assortment c. law of continuity d. law of segregation

3. The way in which an organism appears is its a. genotype b. phenotype c. pleiotropy d. codominance

4. If two heterozygous parents mate both carriers for a recessively inherited form of porphyria) what is the chance that their offspring will have porphyria? a. 0 b. 25 c. 50 d. 100

5. Which stage of meiosis involves the separation of homologous chromosomes? a. anaphase I b. anaphase II c. prophase I d. prophase II

6. Which represents the correct flow of stages in meiosis? a. prophase II➔metaphase I➔anaphase I➔telophase I b. prophase I➔metaphase I➔anaphase I➔telophase I c. anaphase II➔prophase II➔telophase II➔metaphase II d. telophase I➔anaphase I➔metaphase I➔prophase I

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(a) (b)

7. Which is the source of hemophilia for Prince Frederick using the pedigree in the figure below? a. grandmother b. grandfather c. mother d. uncle

8. In the Hardy–Weinberg equation, if the frequency of recessive alleles is 5% of the population, what is the number of recessive individuals in that population? a. 25 out of 10 b. 25 out of 100 c. 25 out of 1,000 d. 25 out of 10,000

9. In question #8 above, what is the frequency of dominant genes in the population? a. 5% b. 25% c. 75% d. 95%

10. Which statement best describes the benefits of GMOs to society? a. Photosynthesis decreases greenhouse gas effects. b. The food supply can support the population. c. Nonnative species are kept in check. d. GMOs kill many species of insects.

short answers

1. Describe how porphyria affects the health of those inheriting it. Describe the mechanism by which porphyria causes damage. How does porphyria get portrayed as vampirism in history? Is it justified? Why or why not?

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2. Define the following terms: phenotype, genotype, and pleiotropy. List one way each of the terms differ from each other in relation to heredity. Give an example found within fowl to make this clarification.

3. Describe the experiments of Gregor Mendel leading to the law of independent assortment. How does this law relate to genetic diversity within offspring?

4. In question #3, describe two other mechanisms by which genetic diversity is increased in populations through sexual reproduction.

5. Draw a Punnett square for a cross between two heterozygous tongue rollers? What percentage of their offspring are heterozygotes? Homozygous dominant?

6. List the stages of meiosis I and II, indicating the point at which a cell becomes haploid. Why does it become haploid at this point?

7. Describe a testcross used to determine genotype in a pedigree. What is an advantage of a testcross to determine genotype in a pedigree? Are its results certain? Why or why not?

8. One in 22 people in the United States are carriers for cystic fibrosis. What is the per- centage of individuals who actually have this disease, using the Hardy–Weinberg equa- tion? Show your work.

9. Describe the process of recombinant DNA technology. Use the following terms to write its description: restriction enzyme, bacterial plasmid, vector, human DNA, protein.

10. Define the process of inbreeding. What are disadvantages of inbreeding? Are there any advantages? Explain your answers genetically.

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Biology and society Corner: Discussion Questions 1. Diseases such a porphyria manifest in ways that give them a bad reputation. What

other inherited traits have a bad reputation in society? Choose one trait and discuss how it is treated by the dominant culture. How are people with the trait treated differently? Suggest ways to improve the lives of people with this trait within our society.

2. The genetic basis of sexual preference was advocated for in this chapter. With which side do you agree, genetic or environmental in cause? What factors do you think limit or enhance the acceptance of alternative sexual preferences in society? Does the idea of a genetic basis have an impact in this acceptance?

3. If a society decided to remove all of the harmful recessive genes, such as cystic fibrosis, within its population, what would be its ethical difficulty? What would be its practical difficulty, based on the Hardy–Weinberg equation? Explain you answer fully.

4. Race is used in decision-making regularly in the U.S. organizations. Why is race such an important factor in society? Do you think it should be so? Is there a genetic basis to human race classifications?

5. A health food guru claims that GMOs are making people fat. Explain why this state- ment is false. How have GMO foods helped society? How have GMO foods harmed society? Is your answer certain? Why or why not?

Figure – Concept Map of Chapter 6 Big Ideas

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Chapter 5: Molecular Genetics��
Chapter 6: Inheriting Genes��