Klinefelter syndrome occurs as a result of a random error that causes a male to be born with an extra sex chromosome. Of the 46 human chromosomes, the two sex chromosomes determine a person's sex. In females, both sex chromosomes are X (written as XX). Males have an X and a Y sex chromosome (XY).

Most often, Klinefelter syndrome occurs because of one extra copy of the X chromosome in each cell (XXY). Extra copies of genes on the X chromosome can interfere with male sexual development and fertility.

Some males with Klinefelter syndrome have the extra X chromosome only in some of their cells (mosaic Klinefelter syndrome). Rarely, a more severe form of Klinefelter can occur if a male has more than one extra copy of the X chromosome.

Klinefelter syndrome isn't an inherited condition. Rather, the additional sex chromosome results from a random error during the formation of the egg or sperm or after conception.

Jun. 25, 2013