What if a white eyed fruit fly were crossed with a heterozygous red eyed female fruit fly what ration of genotypes would be expected in offspring?

Answer 1

white eye gene is ressecive, so the white eyed male is homocigous. You already knew that. So, wether the male apports either of his chromosome set, he will give the white eye gene in it. Now the heterocigous red eyed fly has the red eye gene (which is dominant), and the white eye gene (which is ressecive). But since the red eye gene is dominant, the fly has red eyes. Now, theorycally half the offspring would have white eyes, and the other half would have red eyes, because the male would always apport a white eye gene, cause he has a pair of white eyes genes; the female has a white eye gene and a red one. So, she will apport white gene half the times and red gene half the times, and half the offspring would be homocigote (white eye), and the other half would be heterocigote (red eye, since the red was dominant). To make it clear, lets call the white gene W, and the red one R. So the male set would be: (W, W) while the female would be (W, R), now combining them in all possibilities we have: (W, W), (W, R), (W, W), (W, R), as you can see, of four 2 are white eyed homocigotes and 2 red eyed heterocigotes. By the way, check that that is exactly what happens in humans about the sex, female being (X, X), and male being (X, Y), and so statistically we should have half the offspring being males and half of it being females. note: that applies only if the fly species is diploid, like humans. If drosophila melanogaster, which is the one you are talking about i think, isn't diploid, then it would be different from the above.