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.

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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.



1 Answer 1


Since Drosophila have the same system of sex chromosomes as human beings, (XY males and XX females). Its clear that the sex determination is important - no females have yellow bodies. This is the key observation.

Amongst the males you have about 50%/50% Y/y phenotype. This appears to be a classic sex chromosome related trait. Does this help? I can go further, but this should get you there.

  • 1
    $\begingroup$ Thank you for your answer, it did help. I made the problem more complicated than it is, the answer seems obvious now. $\endgroup$
    – user34914
    May 13, 2014 at 17:35
  • 1
    $\begingroup$ great to hear! it takes some practice to get into this stuff ! $\endgroup$
    – shigeta
    May 13, 2014 at 22:23
  • $\begingroup$ @user34914 Although for your question, you can use the same system of sex chromosomes as human beings' , it actually is more complicated. It also depends on autosomes. See this wikipedia article. $\endgroup$
    – biogirl
    May 15, 2014 at 15:14
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    $\begingroup$ @user34914 Genetics - by Ursula Goodenough is an excellent book for studying classic genetics. $\endgroup$
    – biogirl
    May 15, 2014 at 15:15

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