Sex-linked genes are those carried on the mammalian X chromosome but not the Y chromosome. (In avian species (such as chickens), which have Z and W chromosomes, sex-linked genes are on the Z chromosome, and "male" and "female" are exchanged.) The recessive phenotypes of such genes are more common in males than in females; to be precise, the incidence in females is the square of that in males, so if 1/20 of the male population is green-blind, 1/400 of the female population is. (AIS does not follow this rule because it interferes with reproduction.)

Sex-linked traits are inherited through chains of carrier mothers. That is, a girl, her mother, and her mother's mother all are carriers, while some of their sons have the trait.

Some sex-linked traits in humans:

In cats: