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).
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. The B/b gene doesn't affect the yellow fur color when there is one AY allele. You do not need to consider the A gene in this cross in calculating the probability.
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The unlinked genes that determine the two traits are independent. You need to use the multiplication rule to calculate theprobability of agouti brown offspring 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.
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