Genetics Practice Problems

Genetics Practice Problems #1-

1. In horses, hair color is controlled by a gene A. The dominant allele A produces black hair; the recessive a gives brown. A second gene B controls hair length, with long hair (B) dominant to short hair (b). A homozygous black, short-haired female horse is mated to a homozygous brown, long-haired male. What will be the genotypes and phenotypes of the F1 generation? Of the F2 generation?

2. In shorthorn cattle, the gene for coat color is an example of incomplete dominance (a heterozygous animal is a color intermediate between the two pure gene colors). A breeder of these cattle has cows that are white and a bull that is roan (a mixture of red and white). What fraction of the calves produced in his herd will be white? Roan? Red?

3. Starting with a roan bull and white cows, as in problem 2, could the breeder eventually establish a true-breeding red herd? How?

4. In humans, normal skin pigmentation is due to a dominant gene C; its recessive c results in albinism. A normal man marries an albino woman. Their first child is an albino. What are the genotypes of these three people? If the couple have more children, what are they likely to be?

5. Watermelons may be either plain green or striped in color, the fruit may be either long or round in shape. A watermelon plant of a homozygous long, green variety was crossed with one of a homozygous round striped variety. The F1 plants all bore round, green melons. (a) How many genes are involved in this cross? (b) Which alleles are dominant? (c) If two plants are crossed, what fraction of the F2 generation would be round and striped?

6. What results would you expect if one of the F1 plants in problem 5 was crossed with a plant of a long, striped variety?

7. The following problems all concern tomatoes. Red tomato fruit color is dominant to yellow, and round tomato shape is dominant to oval (pear-shaped). Each characteristic is controlled by a single gene, and the genes are on separate chromosomes. Phenotypes of parent and offspring plants are given in succeeding questions. For each question, determine the genotypes of the parents.

a. Red/round crossed with yellow/oval produced one-half red/round and one-half red/oval.

b. Red/oval crossed with red/oval produced three-fourths red/oval and one-fourth yellow/oval.

c. Yellow/round crossed with red/oval produced all red/round.

d. Red/round crossed with red/oval produced three-eighths red/round, three-eighths red/oval, one-eighth yellow/round, and one-eight yellow/oval.

e. Red/round crossed with red/round produced 56 red/round, 18 red/oval, 19 yellow/round, and 6 yellow/oval.