Eye color in the fruit fly is said to be sex-linked. This simply means that the gene for eye colour is:

Morgan found that the eye color gene was inherited in different patterns by male and female flies. Male flies have an X and a Y chromosome (XY), while female flies have two X chromosomes (XX).
The first white-eyed fly he found was male, and when this fly was crossed with normal, red-eyed female flies, the offspring were all red-eyed—telling Morgan that the white allele was recessive.
But when the F1 flies were crossed to each other, something strange happened: all of the female F2 flies were red-eyed, while about half of the male F2 flies were white-eyed.
Clearly, the male and female flies were inheriting the trait in different patterns. In fact, they were inheriting it in the same pattern as a particular chromosome, the X.