Here's why:

* White eyes in fruit flies are a recessive trait linked to the X chromosome.

* Females have two X chromosomes (XX) while males have one X and one Y chromosome (XY).

* If the white eye allele is on the X chromosome, a male fruit fly will express the trait because he only has one X chromosome. A female fruit fly would need to inherit the white eye allele on both of her X chromosomes to express the trait.

* In the F2 generation, if only males have white eyes, it suggests that the mother was a carrier for the white eye allele (had one X chromosome with the white eye allele and one X chromosome with the wild-type allele). The father would have had the wild-type allele on his X chromosome.

Let me know if you'd like a more detailed explanation of how this inheritance pattern works in the F2 generation!