SRY: The Tiny Switch

On the Y chromosome sits a small gene called SRY. Early in development, SRY usually triggers testes to form. Those testes then release hormones that steer the embryo down a typically male developmental path.

The switch is powerful, but not magic. If SRY is missing or does not work, an XY embryo can develop along a typically female path. If SRY is copied onto an X chromosome, an XX embryo can develop testes.