Klinefelter's syndrome arises when a boy carries an XXY, rather than XY chromosome. The additional female sex X chromosome may be expressed in the adult man through enlarged breasts, sparse facial and body hair, small testes, and he may be unable to produce sperm. Language and reading disabilities and angry mood shifts are also linked to the syndrome.
1 in 500 to 1 in 1000 boys is born with the XXY pattern. About 10% of those affected have enlarged breasts. Surgery and hormone therapy can help.