Klinefelter Syndrom is  a genetic condition that results when a boy is born with an extra copy of the X chromosome. Klinefelter syndrome is a common genetic condition affecting males. Affects testicular growth,smaller than normal testicles, affects the production of the sex hormone testosterone. Klinefelter syndrome may also cause reduced muscle mass, reduced body and facial hair, and enlarged breast tissue. The effects of Klinefelter syndrome vary, and not everyone with it develops these signs and symptoms.