In fruit flies (Drosophila melanogaster) grey body is dominant to black body and normal wings are dominant to vestigial wings. If a heterozygous grey fruit fly is mated with a black-bodied fruit fly, what proportion of the offspring would be black?

A. 0

B. 0.25

C. 0.5

D. 1

The answer is C. We can suppose that the Grey gene is "A" and Black gene is "a". So the gene of heterozygous grey fruit fly is "Aa", and the black-bodied fruit fly is "aa". After mating, the offspring can get only "a" from balck-bodied fruit fly and has equal opportunity to get a "A" or "a" from heterozygous grey fruit fly. If the gene of offspring is "Aa", it will be grey. If "aa", it will be black. So the proportion of being black is 0.5.