A homozygous black Labrador retriever is crossed with a homozygous yellow Labrador retriever. The heterozygous F1 are all black. When these black F1 are intercrossed, the F2 generation displays a phenotypic ratio of 9 black : 3 brown : 4 yellow. Which of the following could represent genotypes of the yellow F2 dogs? Let A and B represent genes at different loci and let the A allele be dominant to a allele; B allele be dominant to b allele. only aabb only aaB A_B_and aaB. aaB_ and aabbr only A_ B_ aaB_ and A_bb.