sex-linked trait

A sex-linked trait is one whose gene rides on a sex chromosome — the X or the Y — rather than on one of the numbered autosomes. Because the two sexes carry different sex chromosomes, such a trait does not pass through families in the even-handed way that an autosomal trait does.

The great majority of sex-linked genes are on the much larger X chromosome, so in everyday usage 'sex-linked' often means 'X-linked.' Since males have a single X and females have two, an allele on the X plays out very differently in the two sexes: a recessive X allele that is masked in females routinely shows up in males, who have no second X for backup.

A handful of genes lie on the small Y chromosome and are therefore passed strictly from father to son (Y-linked, or holandric, inheritance). The defining feature of all sex linkage is that the inheritance pattern depends on the sex of both parent and child, which is exactly what makes these traits stand out on a pedigree.