Klinefelter's syndrome, 47, XXY, or XXY syndrome is a condition in which human males have an extra X chromosome. While females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome.Because of the extra chromosome, individuals with the condition are usually referred to as "XXY Males", or "47, XXY Males". This chromosome constitution (karyotype) exists in roughly between 1:500 to 1:1000 live male births[3][4] but many of these people may not show symptoms. The physical traits of the syndrome become more apparent after the onset of puberty, if at all.
