Question

In addition to A and a, the “agouti” gene has a third allele, AY . Here is some information about the inheritance of the AY allele.
The AY allele is dominant to both A and a.
The homozygous genotype (AYAY ) results in lethality before birth.
The heterozygous genotypes (AYA or AYa) result in yellow fur color, regardless of which alleles are present for the B/b gene. (This effect exhibited by the AY alleleis known as epistasis--when the expression of one gene masks the expression of a second gene.)

Suppose you mate two mice with the genotypes AYaBb x AYaBb . Considering only the live-born offspring, what would be the expected frequency of mice with yellow fur?(For help getting started, see Hint 1.)
Express your answer as a fraction using the slash symbol and no spaces (for example, 1/16).

Answer 1

2/3. Because the presence of the AY allele is epistatic to (masks expression of) the B/b gene, the B/b gene does not need to be taken into consideration inthis problem. For the AYa x AYa cross, 1/4 of the offspring would have the AYAY genotype, which is lethal before birth. For the live-born offspring, 2/3would be AYa, and thus have yellow fur. Because yellow fur color is epistatic (when one AY allele is present), the genotype of the B/b gene would notaffect the phenotype. Thus, you only need to consider the genotype of the A gene in this cross (AYa x AYa) in calculating the probability.

Answer 2

Because the two traits are determined by unlinked genes, they assort independently. As a result, you need to use the multiplication rule to calculate theprobability of agouti brown offspring (A_ bb) from AaBb parents. The probability of A_offspring is 3/4, and the probability of bb offspring is 1/4. Thecombined probability is therefore 3/4 x 1/4 = 3/16.