In humans, the presence of a Y chromosome determines gender and makes the individual male.

• Turner syndrome (X_) individuals are females: missing the small Y is not serious.

• Klinefelter syndrome (XXY) individuals are males: the extra X is inactivated.

• XYY individuals are males: an extra small Y is not too harmful.

• XXX individuals are females: the two extra X are inactivated.

Calico cat and X inactivation

• Can a calico cat be male?
  • Yes, if he is Klinefelter (XXY).