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 chromosome 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 testes are small (about half typical size) and quite firm. After puberty, the ejaculate contains no sperm. Other effects of Klinefelter are quite variable. Boys with Klinefelter are usually born with male genitals that look like other boys. But at puberty, they may not virilize very strongly—they may not develop much body hair, or they may experience breast development. If the boy \*wishes\* to virilize, testosterone (either through injections or via patches) can help him to do so.
Although most boys with Klinefelter Syndrome grow up to live as men, some do develop atypical gender identities, and some do develop female gender identities.
We’ve collected some online resources about Klinefelter Syndrome.
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