48, XXYY syndrome is a sex chromosome anomaly in which males have an extra X and Y chromosome. Human cells usually contain two sex chromosomes, one from the mother and one from the father. Usually, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The appearance of at least one Y chromosome with a properly functioning SRY gene makes a male. Therefore, XXYY normally only affects males. Males affected with XXYY syndrome have 48 chromosomes instead of the typical 46. This is why XXYY syndrome is sometimes written as 48, XXYY syndrome. It is estimated that XXYY affects one in every 18,000–40,000 male births.