homework

What is a Karyotype?

KKaryotype of a Human Malearyotype of a Human Male

A karyotype is an image in which the chromosomes of a cell have been organized into pairs based on size and shape.

In this example, we can see the 22 pairs of human autosomes, and the X and Y sex chromosomes.

What is the difference between sex-linked and non- sex linked genes?

Human Sex ChromosomesHuman Sex Chromosomes

Sex-linked genes are located on the sex-determining chromosomes, such as the X and Y chromosomes in humans shown here.

What is the difference between sex-linked and non- sex linked genes?

Human ChromosomesHuman Chromosomes

Non-sex linked genes are located on the non-sex determining chromosomes, such as the 22 pairs of human autosomes shown here.

2 What is the difference between sex-linked and non-sex linked genes?

Alleles Are Variations of Genes

In humans, chromosomes exist as pairs; one comes from the mother and the other comes from the father.

Genes are located on the chromosomes and there are often variations of the genes called alleles.

In this diagram, there is a chromosome from the mother in red and a chromosome from the father in blue. Each chromosome has four genes and there are two alleles for each gene. One allele is represented by an upercase letter, and the other allele is represented by a lower case allele.

What is a dominant allele?

A dominant allele is an allele whose phenotypic effect is expressed over that of other alleles when present in one or two copies.

This means that a dominant allele is always expressed, or dominates, when present.

Achrondoplasia is a form of short-limbed dwarfism. Almost all cases of this phenotype result from a dominant mutation in a gene located on chromosome 4. Inheritance of two copies of the mutant allele is often fatal.

In this example, as shown in the left image, and individual who inherits one or more copies of the dominant achrondoplasia allele has a short stature. These genotypes that can result in this phenotype are written as “AA or “Aa”.

Normal height is only observed in individuals with the homozygous recessive, or “aa”, genotype.

What is a recessive allele?

Dominant vs. RDominant vs. Recessive Allelesecessive Alleles

A recessive allele is an allele whose phenotypic effect is masked, or hidden, by the presence of other alleles for the same gene.

This means that an individual must possess two copies of the recessive allele in order for the phenotypic effect of the recessive allele to be observed.

In this illustration, the individual on the right is expressing the recessive oculocutaneous albinism trait. As shown here, this results from having two copies of the recessive albinism gene, written as mm.

The individual on the left, has the normal pigmentation phenotype. This is due to having one or two copies of the dominant normal allele.

Chromosomal Structure

Humans have two copies of each chromosome. One member of each pair was contributed by one parent.

The members of each chromosome pair are referred to as homologous chromosomes. The illustration contains one pair of homol- ogous chromosome, the purple colored chromosome and the green colored chromosome.

Prior to division, cells must duplicate their chromosomes. The two copies of a particular chromosome are called sister chromatids. The illustration contains two sets of sister chromatids, purple and green.

Until the sister chromatids are separated into the appropriate daughter cells, they are held together by a structure called a cen- tromere.

What is the function of meiosis?

The Production of Gametes By MeiosisThe Production of Gametes By Meiosis

Meiosis is the process by which the number of chromosomes is reduced by half during the formation of gametes in sexually reproducing organisms.

In this example, a parent cell has two different chromosomes, represented by the colors red and blue. The parent cell has two copies of each chromosomes, one from each of its parents.

We can represent the total number of chromosomes in the parent cell with the formula 2n = 4. “n” equals the number of different chromosomes, and the “2” indicates that there are two copies of each. Thus the total number of chromosomes in the parent cell is 4.

Please note that the parent cell has already replicated its chromosomes in preparation for meiosis. When counting chromosome numbers in a cell, sister chromatids are counted as a single chromosome.

At the completion of meiosis, gametes have been formed and the number of chromosomes in a cell is 2. This is represented by the formula n = 2.

The gametes contain only one copy each of the parental chromosomes.

Comparing Meiosis I to Meiosis II

Differences Between Meiosis and MitosisDifferences Between Meiosis and Mitosis

Meiosis is composed of two rounds of cell division and is divided into Meiosis I and Meiosis II. Let us compare the major differ- ences between these two stages.

Two important events occur in Meiosis I. First, an event called crossing-over allows for the exchange of genetic information between non-sister chromatids of homolog pair.

Secondly, the homologs themselves are separated to different daughter cells. This means that the individual chromosomes of a homologous pair are segregate into different daughter cells.

In Meiosis II the sister chromatids are segregated into different daughter cells. These daughter cells have half the number of chro- mosomes as the original parent cell.

What is a pair of homologous chromosomes?

© CNRI/SPL/Photo Researches, Inc.

Humans have 23 pairs of chromosomes.

Except for the X and Y sex chromosomes, each member of a pair share identical lengths, position of the centromere, staining patterns, and genes for the same characteristic at corresponding locations.

In this example, chromosome pair number 5 is highlighted.

One member of each pair was contributed by the female parent, and the other was contributed by the male parent.

Each of these chromosome pairs are referred to as homologous chromosomes or homologues.

Punnett square for human sex-chromosomes

To set up a Punnett square you must segregate the genetic information of each parent into gametes. This information is contained in the provided parental genotypes.

In this example, the components of the genotype for each parent are the two copies of the sex-chromosomes. In other examples, the genotype may contain specific alleles for one or more genes.

The genotype of the male parent is “XY”. The two boxes along the left side of the Punnett square are for the two types of sperm that can be produced with respect to this genotype. Put an “X” in one box, and a “Y” in the other box as shown.

The genotype of the female parent is “XX”. The two boxes along the top of the Punnett square are for the two types of eggs that can be produced with respect to this genotype. Put an “X” in one box, and an “X” in the other box.

What Is Nondisjunction?

Nondisjunction in Meiosis INondisjunction in Meiosis I

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis. This results in the pro- duction of gametes with an incorrect number of chromosomes.

Let's first look at the effects on gamete production when nondisjunction occurs in Meiosis I. For simplicity, only one chromosome is shown in the illustration.

In Meiosis I, nondisjunction results when a pair of homologous chromosomes do not separate from each other during Anaphase I.

We can see that for the two daughter cells produced in Meiosis I, the cell on the left has two homologous chromosomes, instead of the normal one homologous chromosome. The cell on the right is missing this homologous chromosome.

This is followed by Meiosis II in which the sister chromatids separate and a total of four daughter cells, two with an extra chromo- some (n+1), and two that are missing a chromosome (n-1).

What is nondisjunction?

Nondisjunction in Meiosis IINondisjunction in Meiosis II

In the four daughter cells, two have a normal number of chromosomes, one has an extra chromosome, and one is missing a chro- mosome.

In Meiosis II, nondisjunction results when a pair of sister chromatids do not separate from each other during Anaphase I.

In this illustration, Meiosis I proceeds normally and both daughter cells have a homologous chromosome composed of sister chromatids.

In Meiosis II, nondisjunction occurs in the daughter cell on the right where the sister chromatids fail to separate.

2 What is nondisjunction?

What is a phenotype?

Human Hairline PhenotypesHuman Hairline Phenotypes

(c)HFPA, 63rd Golden Globe Awards; (c) Dynamic Graphics/ PictureQuest RF

A phenotype is a characteristic that can be observed or measured in an individual.

In this image, two different phenotypes for the human hairline are shown.

What is a genotype?

The term genotype refers to the genetic makeup of an individual.

Although genotype usually refers to the specific alleles that determine a particular phenotype or two, it can also refer to the spe- cific alleles that determine a larger number of phenotypes, up to and including all of the phenotypes of an individual.

There are several forms of albinism, an inherited phenotype that is characterized by a lack of the pigment melanin in the skin, hair, and/or eyes.

Oculocutaneous albinism type 1, as shown in the right image, is caused by a recessive mutation in chromosome 11. The genotype for this condition is two copies of the recessive allele, written as “mm”.

Individuals with the genotype “MM” or “Mm” have normal pigmentation.

Defining homozygous and heterozygous

(c)HFPA, 63rd Golden Globe Awards; (c) Dynamic Graphics/ PictureQuest RF

The terms homozygous and heterozygous refer to the level of similarity between two alleles in a genotype.

In this example, the “WW” and “ww” genotypes each contain identical alleles and are referred to as homozygous.

To differentiate between the two possible homozygous genotypes, they are called homozygous dominant when composed of the two copies of the dominant allele, and homozygous recessive when composed of two copies of the recessive allele.

The “Ww” genotype has two non-identical alleles and is referred to as heterozygous.

What is a sex-linked gene?

The term sex-linked refers to genes found on one sex chromosome but not on the other.

In humans there are two types of sex-linked genes . X-linked genes are found only on the X chromosome, and Y-linked genes are found only on the Y chromosome.

Both males and females express X-linked traits, but only men can express Y-linked traits.

What is hemizygous?

Human Sex ChromosomesHuman Sex Chromosomes

Hemizygous refers to the condition where an individual has only one copy of a given gene.

As an example, the human Y chromosome is much shorter than the X chromosome which means that many of the genes located on the human X chromosome are not found on the Y chromosome. Therefore human males, with only one copy of the X chromo- some, are hemizygous for several genes.

In this illustration, the male individual has only one copy of the C gene and is said to be hemizygous for this gene.

What is a pedigree?

PPedigree Chart for Huntington Diseaseedigree Chart for Huntington Disease

A pedigree is a family tree that shows the occurence of a particular trait in parents and offspring for several generations.

Conventional symbols are used in a pedigree chart such as a box to represent males, and a circle to represent females. Colored boxes or circles indicates an affected individual.

In this example, we see the pattern of occurence of an inherited disease in a family. Huntington disease is caused by a dominant allele. Note that each affected offspring in this pedigree has an affected parent.