Most chromosomes look the same in males and females. Sex chromosomes are the exception; they differ between the sexes and carry the primary signals that determine sexual development. In humans the pair is XX in females and XY in males.

The X chromosome is large and gene-rich; the Y is smaller and carries the SRY gene that triggers male development. Other species use different systems—ZW in birds, for example. Sex chromosomes evolved from ordinary autosomes and still share residual homology in small pseudoautosomal regions that pair during meiosis.

Genes on sex chromosomes show distinctive inheritance patterns. Dosage compensation mechanisms equalize X-linked gene expression between males and females. Abnormalities in sex-chromosome number produce conditions such as Turner and Klinefelter syndromes. Studying these chromosomes reveals both the mechanics of sex determination and the evolutionary history of sexual dimorphism.

• Chromosome involved in sex determination
• XX/XY system in humans
• X is large and gene-rich; Y carries SRY
• Shows unique inheritance and dosage effects