Affected individuals typically have small testes that do not produce as much testosterone as usual. A shortage of testosterone can lead to delayed or incomplete puberty, breast enlargement (gynecomastia), reduced facial and body hair, and an inability to have biological children (infertility). Some affected individuals also have genital differences including undescended testes (cryptorchidism), the opening of the urethra on the underside of the penis (hypospadias), or an unusually small penis (micropenis).
Most often, Klinefelter syndrome results from the presence of one extra copy of the X chromosome in each cell (47,XXY).
