What is klinefelter syndrome?
Klinefelter syndrome, also known as the XXY condition, is a term used to describe males who have an extra X chromosome in most of their cells. Instead of having the usual XY chromosome pattern that most males have, these men have an XXY pattern. Klinefelter syndrome is named after Dr. Henry Klinefelter, who first described a group of symptoms found in some men with the extra X chromosome. Even though all men with Klinefelter syndrome have the extra X chromosome, not every XXY male has all of those symptoms. Because not every male with an XXY pattern has all the symptoms of Klinefelter syndrome, it is common to use the term XXY male to describe these men, or XXY condition to describe the symptoms. Scientists believe the XXY condition is one of the most common chromosome abnormalities in humans. About one of every 500 males has an extra X chromosome, but many don’t have any symptoms. For more information on genes and chromosomes, check out Cells 101.
In 1942, Dr. Harry Klinefelter and his coworkers at the Massachusetts General Hospital in Boston published a report about nine men who had enlarged breasts, sparse facial and body hair, small testes, and an inability to produce sperm. By the late 1950s, researchers discovered that men with Klinefelter syndrome, as this group of symptoms came to be called, had an extra sex chromosome, XXY instead of the usual male arrangement, XY. (For a more complete explanation of the role this extra chromosome plays, see the accompanying section, “Chromosomes and Klinefelter syndrome.”) In the early 1970s, researchers around the world sought to identify males having the extra chromosome by screening large numbers of newborn babies. One of the largest of these studies, sponsored by the National Institute of Child Health and Human Development (NICHD), checked the chromosomes of more than 40,000 infants.
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.
Klinefelter syndrome is a condition that occurs in men as a result of an extra X chromosome. The most common symptom is infertility. Humans have 46 chromosomes, which contain all of a person’s genes and DNA. Two of these chromosomes, the sex chromosomes, determine a person?s gender. Both of the sex chromosomes in females are called X chromosomes. (This is written as XX.) Males have an X and a Y chromosome (written as XY). The two sex chromosomes help a person develop fertility and the sexual characteristics of their gender. Most often, Klinefelter syndrome is the result of one extra X (written as XXY). Occasionally, variations of the XXY chromosome count may occur, the most common being the XY/XXY mosaic. In this variation, some of the cells in the male’s body have an additional X chromosome, and the rest have the normal XY chromosome count. The percentage of cells containing the extra chromosome varies from case to case.
Most men inherit a single X chromosome from their mother, and a single Y chromosome from their father. Men with klinefelter syndrome inherit an extra X chromosomes from either father or mother; their karyotype is 47 XXY. Klinefelter is quite common, occuring in 1/500 to 1/1,000 male births. The effects of klinefelter are quite variable, and many men with klinefelter are never diagnosed. The only characteristic that seems certain to be present is small, very firm testes, and an absence of sperm in the ejaculate, causing infertility. Except for small testes, men with klinefelter are born with normal male genitals. But their testes often produce lower than average quantities of testosterone, so they don’t virilize (develop facial and body hair, muscles, deep voice, larger penis and testes) as strongly as other boys at puberty. Many also experience some gynecomastia (breast growth) at puberty. Physicians recommend that boys with klinefelter be given testosterone at puberty, so that they wi
