Klinefelter syndrome

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Klinefelter syndrome is the presence of an extra X chromosome in a male.

47 X-X-Y syndrome

Humans have 46 chromosomes. Chromosomes contain all of your genes and DNA, the building blocks of the body. Two of these chromosomes, the sex chromosomes, determine if you become a boy or a girl. Females normally have two of the same sex chromosomes, written as XX. Males normally have an X and a Y chromosome (written as XY).

Klinefelter syndrome results in males who have at least one extra X chromosome. Usually, this occurs as a result of one extra X (written as XXY).

Klinefelter syndrome is found in about 1 out of every 500-1,000 newborn males. Women who have pregnancies after age 35 are slightly more likely to have a boy with this syndrome than younger women.

Review Date: 12/11/2006
Reviewed By: Brian Kirmse, MD, Mount Sinai School of Medicine, Department of Human Genetics, New York, NY. Review provided by VeriMed Healthcare Network.