Females carry two X chromosomes; males carry one. The X chromosome is a large, gene-rich sex chromosome that contains roughly a thousand genes involved in everything from cognition to blood clotting. Because males are hemizygous, X-linked mutations often produce visible phenotypes in sons.

In female cells one X chromosome is randomly inactivated to equalize gene dosage with males. The inactive X forms a Barr body and is silenced by epigenetic mechanisms. Some genes escape inactivation and are expressed from both X chromosomes.

Abnormalities in X-chromosome number cause Turner syndrome (X0) and triple-X syndrome. The chromosome’s unique biology—dosage compensation, sex-linked inheritance, and evolutionary history—makes it a continuing focus of genetic and medical research.

• Gene-rich sex chromosome present in both sexes
• Two copies in females, one in males
• Subject to X-inactivation in females
• Carries many disease-related genes