Review Genetics –Sample Problem Answers

1. Monohybrid Cross: A plant that is heterozygous for the tall gene is self-pollinated. What are the phenotypes of the offspring and in what proportion?

Tt x Tt

75% tall, 25% short

2. Dihybrid Cross: In plants, round seeds is dominant to wrinkled seeds and tall is dominant to short. Show the cross between two plants that are heterozygous for both traits. What are the phenotypes of the offspring and in what proportion?

RrTt x RrTt
9/16 Round, tall
3/16 round, short
3/16 wrinkled, tall
1/16 wrinkled, short

b. What if the second parent was recessive for both traits? What phenotypes would you expect in the offspring?

RtTt x rrtt
1/4 round tall
1/4 round short
1/4 wrinkled tall
1/4 wrinkled shor

3. Epistatic Cross: In Labradors, the yellow coat color is epistatic. Labs can be black (dominant), brown (recessive) or yellow which is caused by a pair of recessive alleles (yy) A heterozygous black lab (BbEe) is crossed with a yellow lab (Bbee). What proportion of the offspring will be black? _____ Yellow? ____ Brown? ____

4/8 yellow, 3/8 black, 1/8 brown

4. Codominance: In cattle, coat color can be red or white. If a red cow is crossed with a white cow, the offspring is a mottled red & white - coloration farmers call "roan". What phenotypes would you get from a cross between a roan and a white cow?

1/2 white, 1/2 roan

5. Blood types: If one parent has type A blood and another parent has type B blood, what are ALL the possible blood types of the children. You do not know the parents' genotypes.

A, B, AB, O

6. Sex Linked: In humans, colorblindness is a sex linked, recessive trait. If a woman who is a carrier for colorblindness marries a colorblind man, what are the chances that their children will be colorblind?

1/2 (1/4 male, 1/4 female)

7. A hairy-stemmed (HH) purple flowered (PP) plant is crossed with one that is smooth-stemmed (hh) and white flowered (pp). Show the genotypes and phenotypes of the F1 generation.

The offspring is crossed with a double recessive test cross. The following results were obtained.

Hairy, Purple	Hairy, white	Smooth, Purple	Smooth, white
45	5	5	45

Explain these results, include an explanation for recombination. The two middle traits represent the recombinants. The expected ratio for a linked cross would be 50/50 (assuming the genes are linked on the same chromosome). The two recombinants occur because crossing-over occurred between the two genes.

Determine the distance between the stem and flower alleles. 10% recombinant frequency means that the alleles are 10 map units apart.

8. The follow crossover frequencies were determined from various experiments: The crossover frequence between gene A and C is 30%. The crossover frequence between B and C is 44%. How far apart are A and B? Sketch the chromosome map.

B========14MU=========A================ 30 MU====================C