Y chromosome sex-determining region: The region on the Y chromosome that decides the sex of the individual. Abbreviated SRY. SRY is necessary for male determination. It encodes the testis-determining factor. Mutations in SRY are responsible for XY females with gonadal dysgenesis who appear to be normal females at birth but at puberty do not develop secondary sexual characteristics (such as breasts), do not menstruate, and have scarred (fibrous) gonads without eggs.