Article: Klinefelter syndrome

What is Klinefelter syndrome?

Klinefelter syndrome is a chromosomal condition that affects male sexual development. Most males with Klinefelter syndrome have one extra copy of the X chromosome in each cell. Some males with the condition have low levels of the hormone testosterone beginning during puberty, which can lead to breast development (gynecomastia) and an increased risk of breast cancer, reduced facial and body hair, and an inability to father children (infertility). Boys with Klinefelter syndrome may also have difficulty with speech and language development.

Variants of Klinefelter syndrome, which involve more than one extra X chromosome or extra copies of both sex chromosomes in each cell, tend to have more severe signs and symptoms. These disorders affect male sexual development but may also include decreased IQ, distinctive facial features, skeletal abnormalities, poor coordination, and more severe problems with speech.

How common is Klinefelter syndrome?

The most common form of Klinefelter syndrome, which is caused by one extra copy of the X chromosome, affects 1 in 500 to 1,000 males. Variants of Klinefelter syndrome are much rarer, occurring in 1 in 50,000 or fewer male births. Females are not affected by Klinefelter syndrome.

What are the genetic changes related to Klinefelter syndrome?

Klinefelter syndrome is a chromosomal condition related to chromosomes X and Y.

Klinefelter syndrome is caused by the presence of one or more extra copies of the X chromosome in a male's cells. Extra copies of genes on the X chromosome interfere with male sexual development, preventing the testicles from functioning normally and reducing the levels of testosterone.

Most often, Klinefelter syndrome is caused by a single extra copy of the X chromosome, for a total of 47 chromosomes per cell. Males normally have one X chromosome and one Y chromosome in each cell (XY), but males with Klinefelter syndrome have two X chromosomes and one Y chromosome (XXY). Some males with Klinefelter syndrome have the extra X chromosome in only some of their cells; these cases are called mosaic XY/XXY.

Variants of Klinefelter syndrome are caused by several extra copies of the X chromosome or extra copies of both the X and Y chromosomes in all of the body's cells. The signs and symptoms of these conditions typically become more severe as the number of X chromosomes increases.

Can Klinefelter syndrome be inherited?

This condition is not inherited, but usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction can result in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain one or more extra copies of the X chromosome as a result of nondisjunction. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have one or several extra X chromosomes in each of the body's cells.

Mosaic XY/XXY occurs as a random error during cell division early in fetal development. As a result of this error, some cells have one X chromosome and one Y chromosome (XY), and other cells have an extra copy of the X chromosome (XXY).