This revision: 26th April 2013 .
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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.
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.
Cognitive and developmental
Some degree of language learning or reading impairment may be present, and neuropsychological testing often reveals deficits in executive functions, although these deficits can often be overcome through early intervention.
There may also be delays in motor development which can be addressed through occupational therapy. XXY males may sit up, crawl, and walk later than other infants; they may also struggle in school, both academically and with sports.