What causes Klinefelter syndrome?

Updated: Mar 23, 2020
  • Author: Germaine L Defendi, MD, MS, FAAP; Chief Editor: Luis O Rohena, MD, MS, FAAP, FACMG  more...
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In 1959, Klinefelter syndrome was found to be caused by a supernumerary X chromosome in a male. [34]

The 47,XXY karyotype of Klinefelter syndrome spontaneously arises when paired X chromosomes fail to separate (nondisjunction in stage I or II of meiosis, during oogenesis or spermatogenesis). [35] Maternal and paternal meiotic nondisjunction each account for approximately 50% of Klinefelter syndrome cases. Seventy-five percent of maternal nondisjunction cases are caused by meiosis I errors, which are associated with increased maternal age. Increased paternal age has been linked to a possible increased risk of Klinefelter syndrome. [36]

Postfertilization nondisjunction is responsible for mosaicism, which is seen in approximately 10% of Klinefelter syndrome patients. Men with mosaicism are less affected and are often not diagnosed. [6]

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