BSC2-HumanGenetics

R

R

Ear lobes: Attached earlobes come straight out horizontally from the skull, and free earlobes hang down on the ear, beneath the point of attachment to the skull. Let “F” represent the dominant allele in your genotype. Attached earlobes are homozygous recessive.

Farsightedness: Farsightedness (when you need glasses to read up close) is caused by a dominant allele, and “normal” sight is recessive. Let “S” represent the allele for farsightedness.

Nearsightedness: Nearsightedness (when you need glasses for better distance vision) is caused by a completely different gene than farsightedness. Here, “normal” sight is the dominant allele. Let “n” represent the allele for nearsightedness.

Widow’s peak: If you have a pointed contour of the hairline on your forehead, you have a “widow’s peak” (think Eddie Munster from the TV show). The trait is caused by a dominant allele, represented “W”. The lack of a widow’s peak is controlled by the recessive allele (w).

Taste: Certain chemicals can only be tasted if you have a particular genotype. An example is phenylthiocarbamide (PTC), which has an unpleasantly bitter taste to people who have the dominant allele (T). If you are unable to taste this chemical, you are homozygous recessive.

Clasped hands: When you clasp your hands together in your lap, you consistently put either the left thumb on top, or the right. This is not the same as being right- or left-handed. The recessive trait is if the right thumb is clasped on top. Use “L” to represent the dominant allele.

Hitchhiker’s thumb: Scientifically known as “distal hyperextensibility of the thumb,” hitchhiker’s thumb is an autosomal recessive trait (h) in which placing the hand in a “thumbs-up” position results in an angle of nearly 90°.

Mid-digital hair: The presence of hair on the back of the middle segment of at least one finger is a dominant trait (M). The absence of hair on the middle segment of all fingers is a recessive trait (m). A single pair of alleles does not cause this trait, so you may have mid-digital hair on some fingers but not others.

Cleft chin: Some people have a prominent dimple or crease in the front of the chin, called a cleft chin; others do not. Use “C” to indicate the presence of a cleft chin, which is dominant to a smooth chin.

Pigmented iris: Melanin is a dark brown pigment found in the skin, and is also the substance that produces the eye colors specified by the genes. The amount and placement (front or back of the iris) of the melanin produces the different eye colors that we see. Our genes tell enzymes how much melanin to deposit in the iris and in which part to place it. When melanin is deposited in the front of the iris, eye color ranging from green to brown results (P), dependent upon the amount of pigment deposited. When melanin is deposited in the back layer only, blue eyes result (p).

Toe length: In some people, the big toe is longer than the second toe (b), while other people have the big toe shorter than the second toe (B).

Exercise 1: Monohybrid Crosses

For each of the traits shown, determine and record your phenotype in Table 1. Based on your phenotype, determine your genotype. Be sure to list all possibilities.

Table 1. Phenotypes and Genotypes

Exercise 2: Dihybrid Cross

The study of inheritance patterns in organisms that differ in two traits is called a dihybrid cross. Mendel conducted dihybrid crosses to determine if different traits of pea plants, such as flower color and seed shape, were inherited independently or together. We now know that the traits are inherited independently (with the exception of linked genes) because members of an allele pair separate from one another during gamete formation. This is known as Mendel’s law of independent assortment.

A cross of two individuals, both of which are heterozygous for the two traits being studied, would look like this:

RrYy x RrYy

Each individual is capable of producing gametes that contain either the dominant or recessive allele for shape (R or r) and the dominant or recessive allele for color (Y or y).

RY Ry rY ry

RY

Ry

rY

ry

The offspring probabilities for this cross are:

· 9/16 having both dominant parental phenotypes (round seed shape, yellow seed color)

· 3/16 having one dominant parental phenotype and one recessive (round seed shape, green seed color)

· 3/16 having the other dominant parental phenotype and the other recessive (wrinkled seed shape, yellow seed color)

· 1/16 having both recessive parental phenotypes (wrinkled seed shape, green seed color)

This 9:3:3:1 ratio is common for dihybrid crosses involving non-linked genes.

Carry out the following dihybrid cross. Be sure to show your work in the 4x4 Punnett square provided.

B. Two individuals who are doubly heterozygous for Widow’s peak and earlobe traits (WwFf) marry and have children.

WF

WF

What the chance is that their children will have both attached earlobes AND no widow’s peak? ____________

Exercise 3: Codominance

Some gene alleles exhibit codominance. Instead of one gene allele (the recessive allele) being hidden by the expression of another gene allele (the dominant allele), both alleles are expressed at the same time. Blood groups are an example of codominance of alleles in humans.

Blood groups: There are four main blood types in humans: O, A, B, and AB. These blood groups vary in the carbohydrate molecules (antigens) that are attached to the surface of the red blood cells (RBCs); these make up part of the surface receptor proteins (glycoproteins) on RBCs. If you have the A allele, you produce the A antigen on your red blood cell (Type A blood). If you have the B allele, you produce the B antigen (Type B blood). If you have the A allele and the B allele, you will produce both (thus you will have type AB blood). If you have neither the A nor B alleles, you carry two O alleles and will not produce either A or B antigen, resulting in type O blood.

Blood always has antibodies in it that react against the carbohydrates that are not normally present ( i.e., people with blood group A have the A carbohydrate on their RBCs, so they would have antibodies to B in their blood). This is shown below (“I” stands for “immunoglobulin”, which is what antibodies really are):

Phenotype Genotype Receptor on RBC Antibodies

O ii none A & B

A IAIA or IAi A B

B IBIB or IBi B A

AB IAIB A & B none

This is completely different from the Rh factor, which is either positive or negative. Usually, children have the same Rh factor as their mother.

4. Blood type: Phenotype? _______________ Genotype? _______________

Example- A woman with type O blood marries a man with type AB blood. Which blood groups are possible for the children? ____A and B____

IA IB

i

i

C. Assume that you are blood group B, your mother is blood group O, and your spouse is blood group A (genotype IAIA). Which blood groups are possible for the children? _______________________ Use the Punnett square to support your answer.

IA

IA

D. Blood types can be used to prove that a male is not the father of a child, but cannot be used alone to prove that he is the father (much more genetic data is needed). Which of the following males might be the father in the following situations: Mother group A, child group B, Male #1 group O, Male #2 group AB? Use the Punnett square to support your answer.

IA

1