XYY is a trisomy in which the male receives an extra "Y" chromosome.

The incidence of this condition is about 1 per 1000 in human males. Other than being slightly taller and having more severe acne than normal, XYY males are not significantly different from "normal" males. An old study suggested that there were more XYY males incarcerated than expected, but this study has been widely countered as being procedurally flawed.