genetics

Last reviewed 10/2020

The disease is characterized by the presence of an extra X chromosome and a single ‘Y’ chromosome in a phenotypic male (1).

  • the most common karyotype is 47 XXY - around 80-90% of KS cases show this classic  karyotype
  • other sex chromosome variants are less common e.g. -
    • mosaicism - e.g. 47,XXY/46,XY
    • other sex chromosomal aneuploidies e.g. -
      • 48,XXYY and 48,XXXY  - seen in 1 per 17,000 to 1 per 50,000 male births
      • 49,XXXXY - seen in 1 per 85,000 to 100,000 male births (2)

This additional sex chromosome in men is a result of meiotic nondisjunction in either the first or the second meiotic division (caused by abnormal separation of chromosomes or chromatids during the process of meiosis) (1)

  • the supplementary X chromosome is of
    • paternal origin  in 50 to 60%
    • maternal origin in 40 to 50% of cases (3)

Reference: