Klinefelter'
s syndrome, 47, XXY or XXY syndrome is a condition caused by a chromosome aneuploidy. Affected males have an
extra X sex chromosome. The principal effect is small testes development and reduced fertility. A variety of other physical and behavioral differences and problems are common, though severity varies and many
boys and men with the condition have few detectable symptoms. It is the second most common extra chromosome condition, and is named after
Dr. Harry Klinefelter, an endocrinologist at Massachusetts General Hospital,
Boston, Massachusetts, who first described it in 1942. The condition exists in roughly 1 out of every
500 to 1,000 males.