Klinefelter's Syndrome (47,XXY)

Original article by Tom Leach | Last updated on 27/5/2014
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This is the presence of an extra X chromosome in males to give (47,XXY). 



  • 1 in 1000 male live births

Clinical features

Usually diagnosed in childhood when there is:
  • Clumsyness
  • Learning difficulties (usually mild)
  • Typically intellectual ability is reduced y 10-20 IQ points, but still within the normal range. More susceptible to behavioural and psychological problems.
  • Self obsessed behaviour



  • Taller than average
  • Long limbs
  • Gynaecomastia + Infertility + small soft testes (Hypogonadism) (30% of cases)
  • ↑ Risk of
  • Most patients are infertile – there is a lack of sperm in the semen (azoospermia)


  • Testosterone – is often given from puberty onwards, and helps the development of secondary sexual characteristics. It also reduces the long-term risk of osteroporosis.However, puberty is often normal in most cases.


  • Extra X chromosome
  • Equal chance of inheritance from mother or father
    • Maternal cases often associated with advancing maternal age
  • Small proportion of cases are due to mosaicism
  • >2 X chromosomes are sometimes present. These cases are associated with more severe learning difficulties, and more pronounced features of Klinefelter’s.