# Drosophila grey / yellow

I am studying biology by myself via distance-learning. I have been trying to solve the following problem for a while now and hoped you could help me. As this is "homework" I would appreciate hints rather than solutions.

Drosophila has a gene "yellow body", $$y$$, on the X chromosome. The other, dominant, allele is "grey body", $$y^+$$. When crossing female grey bodies with male grey bodies we get 50% grey females, 22% grey males and 28% yellow males.

What is the genotype of the parents?

My work so far:

The genotype must be $$Xy^+X?$$ for the female and $$Xy^+Y$$ for the male. There are two cases to consider: $$?=y^+$$ or $$?=y$$. In the former we get $$\frac{3}{8}$$ grey females and $$\frac{1}{8}$$ yellow females and $$\frac{2}{8}$$ grey and yellow males each; that seems correct except for the yellow females. In the latter, we get twice as many yellow females.

Perhaps the XXyy (yellow females) genotype is lethal but this doesn't help in determining the parents' genotypes.

If $$? = y$$ then cross-over from $$Xy^+\mathbf{Xy} \; x \; \mathbf{Xy^+}Y$$ would not eliminate yellow females.

I don't know of other effects to take into account, so I am stuck.



If you'd like to suggest resources (websites, textbooks $$\ldots$$) pertaining to this subject or on biology at this level in general, I would very much appreciate it.

Thank you.

Edit: I'd like to add an image for anyone having a similar problem. 