Klinefelter syndrome is a genetic disorder that occurs in males who have three or more sex chromosomes. At least two of the chromosomes must be X chromosomes, and one must be a Y chromosome. The condition causes males to have some female-like physical features.

What is going on in the body?

Klinefelter syndrome affects only males. The presence of the Y chromosome assures that male sexual features will be present.

Because males with Klinefelter syndrome don't have the normal number of chromosomes, the body's natural balance is upset. The upset in balance leads to:

Klinefelter syndrome usually does not cause mental retardation, but affected males test slightly lower than their brothers and sisters on IQ testing. They also may have problems talking and hearing.

What are the causes and risks of the condition?

Klinefelter syndrome is caused when a fertilized egg has an extra sex chromosome. Most people have 46 chromosomes in the nucleus of each cell of their bodies. These chromosomes exist in matched pairs, meaning there are 23 pairs. One of these 23 pairs determines whether a person will be a male or a female. This pair is called the sex chromosome pair. The sex chromosomes are designated by letters. The letter X designates the chromosome associated with being female. The letter Y designates the chromosome associated with being male. Most females have two X chromosomes. Most males have an X chromosome and a Y chromosome. If two X chromosomes are present, and there is also a Y chromosome, the person will be a male with the Klinefelter syndrome. This condition is also called 47,XXY. Some people with Klinefelter syndrome have more two X chromosomes and more than one Y chromosome.

The physical features of Klinefelter syndrome are caused by abnormally low production of the male sex hormones.

Klinefelter syndrome is seen in 1 in 1,000 total live births, or 1 in 500 male live births.