sex chromosome

Most chromosomes come in two well-matched copies, but one special pair plays a part in steering an organism toward developing as male or female. These are the sex chromosomes — the members of the genome whose makeup helps determine biological sex.

In humans and many other mammals the sex chromosomes are called X and Y. Individuals with two X chromosomes typically develop as female, while those with one X and one Y typically develop as male, largely because the Y chromosome carries a gene that triggers male development. Unlike the autosomes, the X and Y are quite unequal in size and gene content, so the two sexes do not carry the same dose of the genes on these chromosomes.

Sex-determination systems vary across life. Birds use a different pair, labeled Z and W, with the reverse arrangement; many insects, reptiles and plants use yet other mechanisms, and some species have no sex chromosomes at all. Genes located on sex chromosomes are inherited in distinctive patterns called sex-linked inheritance, which is why some conditions appear far more often in one sex than the other.