The original syndrome described by Klinefelter consisted of GYNAECOMASTIA, testicular ATROPHY and INFERTILITY. Intelligence was unimpaired. Patients have been described who have associated mental defects and striking tallness of stature, but the only constant feature of the syndrome is testicular atrophy with resulting azoospermia and infertility.

The atrophy of the testis is the result of ?brosis, which begins to appear in childhood and progresses until all the seminiferous tubules are replaced by ?brous tissue. Gynaecomastia, mental retardation and eunuchoidism (see EUNUCH; loss of male secondary sexual characteristics – small penis, loss of body hair and a high-pitched voice) may be present. Most patients with Klinefelter’s syndrome have 47 chromosomes instead of the normal 46. The extra chromosome is an X chromosome, so that the sex chromosome constitution is XXY instead of XY. Klinefelter’s syndrome is one of the most common chromosome abnormalities and occurs in 1 in 300 of the male population. Patients with this syndrome show that the Y chromosome is strongly sex-determining: thus, a patient who has an XXY chromosome constitution may have the appearance of a normal male, with infertility the only incapacity, while the loss of a Y chromosome leads to the development of a bodily form which is essentially feminine (see TURNER’S SYNDROME).

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