In Klinefelter's syndrome, there is an extra X chromosome, as shown here, with a 47, XXY karyotype. A non-dysjunctional event in meiosis (maternal or paternal) left two X chromosomes in an ovum or a sperm. Affected males are usually normal, though they may be tall and have small testes. Infertility results from absent sperm. About half have gynecomastia. About 10% of cases are mosaics and are less affected.