Klinefelter's syndrome is a condition caused by a chromosome abnormality in males (specifically, a nondisjunction); sufferers have a pair of X sex chromosomes instead of just one. Human males usually have an XY karyotype. The XXY chromosome arrangement is one of the most common genetic abnormalities, occurring in about 1 in 1,000 male births, but the symptoms that may result from having the extra chromosome are less common. Because of this extra chromosome, individuals with the abnormality are usually referred to as "XXY Males" rather than as "suffering from Klinefelter's syndrome."
In mammals with more than one X chromosome, the genes on all but one X chromosome are barred from being expressed. This happens in XXY males as well as XX females. A few genes, however, have corresponding genes on the Y chromosome and are not barred. These triploid genes in XXY males may be responsible for the symptoms of Klinefelter's syndrome.
In adults, the syndrome is characterised by gynecomastia (enlarged breasts), a rounded body type with abnormal body proportions, sparse facial and body hair, small testes, and an inability to produce sperm. Most XXY males have some degree of language impairment as well. The syndrome is associated with an increased risk of breast cancer, pulmonary disease, varicose veins, and osteoporosis.
The condition was identified in 1942 by Dr. Harry Klinefelter in Boston. The cause was not found until the 1950s. The condition is incurable, but its symptoms can be treated in a number of ways, including testosterone treatment and other therapies.
While the gender identity of people with XXY karyotype is usually male, the number of people with a female gender identity among them seems to be higher than could statistically be expected if those cases were indeed, as the current medical opinion assesses, mere coincidences of people having both gender identity disorder and Klinefelter's independently from each other. This obervation however is based on the reports of support groups for transgender and transsexual people, no scientific study on this subject has been done. The fact that a person undergoing treatment for gender identity disorder has Klinefelter's syndrome is often missed or the patient is not told, although in many jurisdiction this additional diagnosis can have legal consequences, for example regarding name change. Also, the medical treatment may have to be adapted.