In addition to A and a, the “agouti” gene has a third allele, AY . Here is some

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 allele is 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.)

Explanation / Answer

In this cross, the B & b alleles don't matter since AY determines what the offspring will look like. So we only need to deal with that gene. A Punnett Square for the cross of AYaBb + AYaBb will look like:

         AY      a

AY   dead   yellow

a     yellow solid*

*Note: the 'solid' result will produce a black or brown mouse depending on the B/b combo that goes with it.

Out of 3 live possibilities, 2 are yellow. This is a 2/3 (66.67%) probability of having a live offspring that have yellow fur.

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