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, and is associated with additional risk for some medical conditions. It is named after Dr. Harry Klinefelter, a medical researcher at Massachusetts General Hospital, Boston, Massachusetts, who first described this condition in 1942.Klinefelter HF Jr, Reifenstein EC Jr, Albright . Syndrome characterized by gynecomastia, aspermatogenesis without a-Leydigism and increased excretion of follicle-stimulating hormone. J Clin Endocr Metab 1942;2:615-624.

Cause

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The XXY chromosome arrangement is one of the most common genetic variations from the XY karyotype, occurring in about 1 in 500 to 1000 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.

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