Klinefelter Syndrome occurs in 1 in 500 to 1 in 1000 live births. People with this disorder develop as males with subtle characteristics that become apparent during puberty. They are often tall and usually do not develop secondary sex characteristics such as facial hair, or underarm and pubic hair.

Men and boys with Klinefelter Syndrome have a Y chromosome and 2 X chromosomes. This is an example of trisomy.

Where does the extra chromosome come from? In about half of Kleinfelter cases, the extra X chromosome is from the mother's egg, while in the other half of cases the extra chromosome is from the father's sperm.