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Klinefelter’s syndrome

Klinefelter’s syndrome is a condition caused by a chromosome nondisjunction in males; affected individuals have a pair of X sex chromosomes instead of just one. It is named after Dr. Harry Klinefelter, a medical researcher at Massachusetts General Hospital, Boston, Massachusetts, who first described this condition in 1942, and is associated with additional risk for some medical conditions.


The XXY chromosome arrangement is one of the most common genetic variations from the XY karyotype, occurring in about 1 in 1,000 male births. Because of the extra chromosome, individuals with the condition are usually referred to as “XXY Males”, or “47,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 symptoms associated with Klinefelter’s syndrome.

Signs and symptoms

XXY males are almost always infertile, and some degree of language impairment may be present. In adults, possible characteristics vary widely and include little to no signs of affectedness, a lanky, youthful build and facial appearance, or a rounded body type with some degree of gynecomastia (increased male breast tissue).

Gynecomastia to some extent is present in about a third of individuals affected, a higher percentage than in the XY population. The far end of the spectrum is also associated with an increased risk of breast cancer, pulmonary disease, varicose veins, and osteoporosis, risks shared with women.

Rare X-linked recessive problems occur even more infrequently in XXY males, since these conditions are transmitted by genes on the X chromosome, and people with two X chromosomes are typically carriers rather than affected.

There are many variances within the XXY, (otherwise known as 47,XXY) population, just like in the 46,XY population. While it is possible to characterise 47,XXY males with certain body types, that in itself should not be the method of identification as to whether someone has 47,XXY or not. The only method of identification is karyotype testing.

The condition was identified in 1942 by Klinefelter in Boston. The cause was not found until the 1950s.


The condition is irreversible, but its symptoms can be altered in a number of ways, including testosterone treatment and other therapies.

While the gender identity of people with XXY karyotype is generally stable, the number of people with gender identity disorder among the whole 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.

The observation on gender identity 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 jurisdictions this additional diagnosis can have legal consequences, for example regarding name change or medical treatment having to be adapted.


  • Klinefelter HF Jr, Reifenstein EC Jr, Albright F. Syndrome characterized by gynecomastia, aspermatogenesis without a-Leydigism and increased excretion of follicle-stimulating hormone. J Clin Endocr Metab 1942;2:615-624.

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