Klinefelter syndrome

Klinefelter syndrome is a chromosomal condition that affects male sexual development. Males with this condition typically have small testes that do not produce enough testosterone, which is the hormone that directs male sexual development before birth and during puberty. A shortage of testosterone during puberty can lead to breast enlargement (gynecomastia), reduced facial and body hair, and an inability to father children (infertility). Older children and adults with Klinefelter syndrome tend to be taller than other males their age. Compared with other men, adult males with Klinefelter syndrome have an increased risk of developing breast cancer and a chronic inflammatory disease called systemic lupus erythematosus. Their chance of developing these disorders is similar to that of normal adult females.

Boys with Klinefelter syndrome may have learning disabilities and difficulty with speech and language development. They tend to be quiet, sensitive, and unassertive, but personality characteristics vary among males with this condition.

Most males with Klinefelter syndrome have one extra copy of the X chromosome in each cell. Variants of Klinefelter syndrome involve more than one extra X chromosome or extra copies of both the X and Y chromosomes in each cell. These conditions tend to have more severe signs and symptoms than classic Klinefelter syndrome. In addition to affecting male sexual development, variants of Klinefelter syndrome are associated with decreased IQ, distinctive facial features, skeletal abnormalities, poor coordination, and severe problems with speech.

Klinefelter syndrome affects 1 in 500 to 1,000 males. Most variants of Klinefelter syndrome are much rarer, occurring in 1 in 50,000 or fewer male births. Klinefelter syndrome does not occur in females.

Klinefelter syndrome is a condition related to the X chromosome and Y chromosome (the sex chromosomes). People typically have two sex chromosomes in each cell: females have two X chromosomes (46,XX), and males have one X and one Y chromosome (46,XY). Most often, Klinefelter syndrome results from the presence of a single extra copy of the X chromosome in each of a male's cells (47,XXY). Extra copies of genes on the X chromosome interfere with male sexual development, preventing the testes from functioning normally and reducing the levels of testosterone.

Some males with Klinefelter syndrome have the extra X chromosome in only some of their cells; in these individuals, the condition is described as mosaic Klinefelter syndrome (46,XY/47,XXY). Individuals with mosaic Klinefelter syndrome may have milder signs and symptoms depending on how many cells have an additional X chromosome.

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.

Read more about the X chromosome and the Y chromosome.

This condition is not inherited; it usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in a reproductive cell 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 more extra X chromosomes in each of the body's cells.

Mosaic 46,XY/47,XXY is also not inherited. It occurs as a random event during cell division early in fetal development. As a result, some of the body's cells have one X chromosome and one Y chromosome (46,XY), and other cells have an extra copy of the X chromosome (47,XXY).