HUMAN GENETICS

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Human Genetics Hands-on labs, inc. Version 42-0059-00-01

Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.

Experiment Summary:

Students will learn how to use Punnett squares and pedigree charts. Students will go beyond classical Mendelian genetics to learn about incomplete dominance, sex-linked traits, and linkage groups.

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ExpErimEnt

ObjEctivEs ● To observe how genotypes influence phenotypes

● To predict a phenotype based on genotype information

● To use Punnett squares to predict mathematical probabilities of offspring

● To use pedigree charts to observe genetic trends in human lineages

Time Allocation: 2 Hours

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Experiment Human Genetics

matErials

MATERiAlS FRoM:

lABEl oR BoX/ BAg: QTy iTEM DESCRiPTioN:

Student Provides 2 Coins 1 Pencil 1 Drawing paper, 1 sheet 1 Set of colored pencils 1 Partner

Note: The packaging and/or materials in this LabPaq may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List form included in the LabPaq.

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Experiment Human Genetics

DiscussiOn anD rEviEw In the late 1800s, Gregor Mendel, the father of the study of genetics, began experimenting with traits of sweet peas. He knew that traits were inherited across generations and was interested in seeing if there was a pattern. His background in physics provided the math needed to test his ideas. His investigations showed that “units of heredity” would separate as each parent went through the process of forming sex cells. These units would recombine in the offspring resulting in the unit of heredity passing from the parent to the offspring. This separation is called the law of independent assortment.

Figure 1: Gregor Mendel, the father of genetics Courtesy of NIH

Today, these “units of heredity” are called genes. Genes are located on chromosomes. Each person has 23 pairs of chromosomes (for a total of 46 chromosomes) present in every cell. Each gene carries the information for the production of proteins. Genes come in alternative forms known as alleles. In simple Mendelian genetics, two different alleles are present, and one allele each is inherited from each parent. One of the alleles is expressed or “turned on” while the other is repressed or “turned off”. The expression of some allele pairs can be complicated, but in this exercise, only simple Mendelian genetics will be studied. The expressed allele is called dominant and the repressed allele is called recessive. The expressed trait is referred to as the phenotype.

Each set of chromosomes are inherited from parents and align with each other as homologous pairs during meiosis. These pairs code for the same proteins or genetic information. The genome is the complete set of all the genes in an organism. genotype refers to the genetic composition of an organism; either as a group of genes or for individual genetic traits.

Dominant and Recessive Alleles

Alleles are usually written as follows: A capital letter signifies a dominant allele, such as “E” for free (unattached) earlobes. A lowercase letter signifies a recessive allele, such as “e” for attached earlobes.

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Experiment Human Genetics

A person’s genotype for a specific trait can be composed of two dominant alleles, two recessive alleles, or one dominant allele and one recessive allele (EE, ee, or Ee). The EE and ee genotypes are homozygous, or the same, as both alleles are either dominant or recessive. The Ee genotype is referred to as heterozygous as there are two different alleles, one for dominant and one for recessive. In these cases, the E will be expressed as free earlobes because it is the dominant trait. The ee is referred to as homozygous recessive; this is the only genotype which will produce the recessive trait, because there are no dominant alleles to mask the presence of this trait. The phenotype is the physical appearance of the expression of the alleles such as free earlobes. When viewing the phenotype, you cannot know if the genotype is EE or Ee. You can only know that E is present. This example is summarized in Table 1.

Table 1: Alleles with associated genotypes and phenotypes

Allele Genotype Phenotype E EE free earlobes

Ee free earlobes e ee attached earlobes

Variations in Allele Expression in Humans

incomplete dominance

This state occurs when both alleles express themselves. Neither allele is dominant nor recessive. An example taken from this exercise is the shape of human hair: curly, wavy or straight. The genotypes for this trait are as follows: D = straight, Dd = wavy, and dd = straight. However, please note that in the case of human hair, this trait is even more complicated than how it is described in this lab exercise.

X-linked

These are genes located on the X chromosome (which is one of the two sex chromosomes). Women have two X chromosomes (their sex chromosome genotype is XX), while men have one X and one Y chromosome (their sex chromosome genotype is XY). The Y chromosome is actually “missing” an arm of a chromosome, so there are no alleles to match with the X. The result is that any allele present on the X chromosome will be expressed in a man regardless of whether it is normally recessive or dominant.

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Experiment Human Genetics

Exercise 1: genotype to Phenotype In the following activity, you will explore the probabilities behind the hereditary process. Using the traits in Data Table 1, you will flip coins to determine the phenotype of offspring. A partner will be needed for this activity.

prOcEDurE 1. Ask another person to help you with this activity.

2. Determine which person will toss for the female and which will toss for the male. There are two alleles per trait, and each parent will contribute one allele.

3. The person who is representing the male will flip a coin to determine the sex of the offspring. A heads-up toss will yield a female offspring; a tails-up toss will yield a male offspring.

4. For all future coin tosses, heads will represent the dominant allele and tails will represent the recessive allele. We will assume that each parent is heterozygous for each trait and therefore can contribute either allele to the offspring.

5. To determine the shape of the face, both people will flip coins at the same time to determine the genotype for the first trait. The coins should be flipped only once for each trait. Each coin flip represents the gamete contributed by the individual parent.

6. Continue to flip the coins for each trait listed in Data Table 1.

7. After each flip, record the trait of the offspring by placing a mark in the appropriate box in the table.

Note: Some information in the table has been simplified as some of the traits are actually produced by multiple genes. Trait 4 (hair type) exhibits incomplete dominance.

8. Combine all of the characteristics into a drawing of the offspring.

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Experiment Human Genetics

Exercise 2: Punnett Squares The probability of genotype and phenotype expression in a new generation of offspring can be predicted mathematically. The use of Punnett squares, developed by the geneticist Reginald Punnett, is a method of organizing the mathematical probabilities of genetic information that will be passed down to subsequent generations. Punnett squares are simple grids that show the genotype of the mother (for one particular gene or trait) on one axis of the gridline, and the genotype of the father on the other axis of the gridline. Each column or row of the grid corresponds to one possible gene to be passed along to the offspring.

The Punnett square for a simple monohybrid cross (examining only one gene or trait of interest) is shown in Figure 2. The father’s genes are shown on the top row, and the mother’s genes are shown in the left column. Note that the connecting cells for each row and column in the grid correspond to each possible gene combination from the father and mother. In this example, each parent is heterozygous (Aa) for the gene or trait.

Figure 2: Simple Punnett square for monohybrid cross

Father ( A a ) × Mother ( A a ) Father ♂

gametes A a

Mother ♀

A AA Aa a Aa aa

When considering a dihybrid cross (examining two genes or traits of interest at one time), all of the options should be shown for the father and mother, as in the monohybrid cross. See Figure 3. The father’s possible gene combinations are shown in the top row, and the mother’s possible gene combinations are shown in the left column. In this example, each parent is heterozygous (AaBb) for each of the two genes or traits.

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Experiment Human Genetics

Figure 3: Punnett square for a dihybrid cross

Father ( A a B b ) × Mother ( A a B b ) Father ♂

gametes AB Ab aB ab

Mother ♀

AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb

If a trait is sex-linked, it is only present on the X or the Y chromosome (recall the discussion of X-linked traits earlier). If the trait is recessive, and is on the X chromosome, then the trait is only displayed in cases where there is only one X chromosome (males) or where two of the X chromosomes display the recessive trait (females). In Figure 4, the father and the mother carry the recessive trait. However, only the father is affected by the recessive trait. When examining the Punnett square, if the father and mother have a daughter, there is a 50% possibility that she will display the recessive trait. In addition, if they have a son, there is also a 50% possibility that he will display the recessive trait.

Figure 4: Punnett square for a sex-linked trait

Father ( Xg y ) × Mother ( XG Xg ) Father ♂

gametes Xg y

Mother ♀

XG XG Xg XG Y Xg Xg Xg Xg Y

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Experiment Human Genetics

Exercise 3: Pedigree charts Mendel’s laws of inheritance cannot be tested as easily with humans as with pea plants. Scientists can control the reproduction of “model organisms” such as pea plants and fruit flies in order to precisely analyze how their traits are inherited. Controlled human reproduction (known as eugenics) is both unethical and illegal. Therefore, when studying lineages of families, using pedigrees (family trees that show phenotypic patterns) is often the only way that geneticists can study patterns of inheritance in humans. Inferences can be made from these lineages. Figure 5 shows an example of a pedigree chart. Note the patterns of the chart. In Figure 5, the trait is caused by an autosomal (non-sex-linked) dominant trait.

Figure 5: Pedigree chart

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Experiment Human Genetics

prOcEDurE 1. Review the pedigree charts in Figures 6 - 8.

2. Using the key provided, label each individual with the correct genotype.

Note: It is helpful to use a blank or a dash to stand in for an unknown allele (as in A- or A_).

The three pedigree charts will exhibit one of the following inheritance patterns:

a. Autosomal Dominant: If at least one chromosome has the allele, the individual will be affected (that it, it will show the phenotype of interest). In a pedigree, one or both parents will be affected, and many of the offspring will also be affected. Individuals with genotypes AA or Aa will be affected; those will aa genotypes will be unaffected.

b. Autosomal Recessive: In order for an individual to be affected, both alleles must be present in the recessive form. This trait does not occur as often as a dominant trait. In a pedigree, affected parents will often have unaffected offspring, and vice-versa.

c. X-linked Recessive: The trait is present only on the X chromosome. In males, only one gene is needed for it to be expressed. In females, the trait must be homozygous in order to be expressed. Females who are heterozygous for this trait will not show have affected phenotypes, but they are carriers of the trait. In a pedigree, the trait will be most commonly seen in males. XAXA and XAXa individuals will be normal females but XAXa individuals will carry the trait. An XAY male will be unaffected, while an XaY individual will be affected.

Figure 6: Pedigree chart 1

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Experiment Human Genetics

Figure 7: Pedigree chart 2

Figure 8: Pedigree chart 3

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Experiment Human Genetics