Klinefelter syndrome


What is Klinefelter syndrome?

Klinefelter syndrome is a chromosome condition that can affect a male's physical appearance, fertility, and cognitive development. Most individuals have two sex chromosomes in each cell that contain some of the instructions for growth and development of the body. Females will have two X chromosomes (46, XX) and males will have one X and one Y chromosome (46, XY). In Klinefelter syndrome, males most often have an extra copy of the X chromosome in each cell (47, XXY). It's the extra X chromosome that interferes with male sexual development and causes the features of Klinefelter syndrome. It's important to note that not all individuals affected with Klinefelter syndrome have the same degree of signs and symptoms.

Males with Klinefelter syndrome typically have small testes, which do not produce as much testosterone as usual. Testosterone is the hormone that directs male sexual development before birth and during puberty. When testosterone is lacking, this can lead to delayed or incomplete puberty, breast enlargement (gynecomastia), reduced facial and body hair, and fertility problems. Some males with Klinefelter syndrome have undescended testes (cryptorchidism), the opening of the urethra on the underside of the penis (hypospadias), or an unusually small penis (micropenis).

Children with Klinefelter syndrome can have delayed speech and language development, as well as learning disabilities. Children affected with Klinefelter syndrome are typically quiet, sensitive, and unassertive, but these characteristics vary among affected individuals.

When males with Klinefelter syndrome enter into adulthood, they tend to be taller than their peers. These males have an increased risk of developing breast cancer and systemic lupus erythematosus (SLE), a chronic inflammatory disease. The chance of an individual with Klinefelter syndrome developing these disorders is similar to that of women in the general population.

To learn more about Klinefelter syndrome, please talk to your genetic counselor or look on a reliable website such as Genetics Home Reference.

Learning about Klinefelter syndrome. National Human Genome Research Institute. October 2011;

Klinefelter syndrome. Genetics Home Reference. January 2013;

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