How does a white-eyed male fruit fly can have a red eyed-eyed father?

Answer 1

This particular trait was shown not only to be recessive, but to lie on the X-chromosome. This is important in this case.

Both male fruit flies in this example only have one copy of the gene, because they only have one X chromosome. If the father has red eyes, it can only be because he has NO versions of the gene that produces white eyes. The son has white eyes, so his only copy must be the recessive white-eyed gene.

Which leaves only two possible sources for the gene: mutations and mom. Of those two events, mutations are so fantastically less likely that most problems won't even consider them.

Mom, on the other hand, being female, has two copies of the eye colour gene. If one was for white and the other not, the recessive gene wouldn't generally show up and you might not know she had it just by looking at her. And, of course, if her eyes are white then ALL her male children are GUARANTEED to have white eyes, since the only way to be male it so get a Y from dad and an X from mom, who would have only copies of the white-eyed gene.

Answer 2

This gene is on the X-chromosome.

R = red, r = white

Male with white eyes has Xr Y.
He got the Y from the father. Y doesn't have the same genes as X has.

He got the X from the mother. Female flies don't even have a Y to give.

Answer 3

If the red eyed father had a recessive allele for white eye.
The mother must also have contributed one allele for white eye, so she could have been white eye herself or another heterozygote for that trait.

Answer 4

if the mother was white-eyed, and if the white-eyed gene is dominant then it's possible for the fly to have white eyes.