In a vial of Drosophila, a research student noticed several female flies (but no male flies) with "bag" wings each consisting of a large, liquid-filled blister instead of the usual smooth wing blade. When bag-winged females were crossed with wild - type males, 1/3 of the progeny were bag-winged females, 1/3 were normal - winged females, and 1/3 were normal - winged males. Explain these results.

The results can be explained if the mutation for bag wings is sex-linked, dominant, and lethal in males.

Explanation:

When a trait is differently expressed in males and females, we must suspect that it's sex-linked.

If we consider the mutation for bag wings as sex-linked and dominant (B), and the wild-type allele (B+) is recessive, then the expected offspring resulting from the cross between a bag-winged female XᴮXᴮ⁺ (heterozygous) and a WT male Xᴮ⁺Y would be:

1/4 XᴮXᴮ⁺ bag-winged female 1/4 Xᴮ⁺Xᴮ⁺ wild-type female 1/4 Xᴮ⁺Y wild-type male 1/4 XᴮY bag winged male

But the bag-winged male doesn't appear in the progeny, and the observed proportions of the other phenotypes are 1/3 each. This suggests that the mutation is lethal in males, and maybe in homozygosis in females as well.