Y chromosome

The Y chromosome is the smaller of the two human sex chromosomes and, in most people who carry it, the switch that sets male development in motion. It is present in typical males alongside a single X, and absent in typical females.

Much of the Y's influence comes down to one gene, SRY, the “sex-determining region Y.” Early in development, SRY acts as a master switch that directs the undifferentiated gonads to become testes; the hormones the testes produce then steer the rest of male development. The Y carries comparatively few genes overall, but several others on it are important for sperm production and male fertility.

Because the Y passes essentially intact from father to son, it traces the paternal line across generations and has become a valuable tool in studying ancestry and population history. Over very long evolutionary timescales the Y has lost many of the genes it once shared with the X, leaving it gene-poor today; whether it will continue to shrink is a matter of ongoing scientific discussion rather than established fact.