CausesBy Mayo Clinic staff
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.
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