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.)