SAMPLE PROBLEMS:

1. Monohybrid Cross: A plant that is heterozygous for the tall gene is crossed with one that is recessive and short. What are the phenotypes of the offspring and in what proportion?

Tt x tt

50% tall, 50% 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. Linkage Group: In fruit flies, the gene for dumpy wings (recessive) is on the same chromosome as the gene for short antennae. If a heterozygous fly (DdAa) is crossed with one that is recessive for both traits (ddaa) are crossed, what are the phenotypes of the offspring and in what proportion? On the first fly, the dominant genes are located on the SAME chromosome.

1/2 DdAa, normal, normal
1/2 ddaa, dumpy short