One third of the black guinea pigs in the F2 generation of the mating of black X brown are homozygous, BB, and the other two thirds are heterozygous, Bb. Since animals cannot, in general, be self-fertilized, how do you think the geneticist can distinguish the homozygous (BB) and heterozygous (Bb) black-coated guinea pigs? He does this by test cross, by mating each black guinea pig with a homozygous brown (bb) guinea pig (Fig.28-5). If all of the offspring are black, what inference would you make about the genotype of the black parent? If any of the offspring are brown, what conclusion would you draw regarding the genotype of the black parent? Can you be more certain about one of these two inferences than the other?