Klinefelter syndrome or Klinefelter's syndrome, also 47,XXY or XXY syndrome, is a genetic disorder in which there is at least one extra X chromosome to a standard human male karyotype, for a total of 47 chromosomes rather than the 46 found in genetically normal humans. While females have an XX chromosomal makeup, and males an XY, individuals with Klinefelter syndrome 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 but many of these people may not show symptoms. If the physical traits associated with the syndrome become apparent, they normally appear after the onset of puberty.
In humans, 47,XXY is the most common sex chromosome aneuploidy in males and the second most common condition caused by the presence of extra chromosomes. Other mammals also have the XXY syndrome, including mice.
Principal effects include hypogonadism and reduced fertility. A variety of other physical and behavioural differences and problems are common, though severity varies and many XXY boys have few detectable symptoms.
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