Klinefelter's syndrome

Definition:

A chromosome abnormality that affects only men and causes hypogonadism.

Causes, incidence, and risk factors:

Klinefelter's syndrome is caused by an extra X chromosome and affects only males. An infant appears normal at birth, but the defect usually becomes apparent in puberty when secondary sexual characteristics fail to develop, and testicular changes occur that eventually result in infertility in the majority of those affected. Some mild cases may go undetected with no abnormalities present except infertility. Increased maternal age may be a risk factor.