How many people are affected by or at risk for Klinefelter syndrome (KS)?

Researchers estimate that 1 in about 500-1,000 newborn males has an extra X chromosome, making KS among the most common chromosomal disorders found in all newborns.1 A third or fourth X chromosome in a male is much rarer.2

Scientists are not sure what factors increase the risk of KS. The error that produces the extra chromosome occurs at random, meaning the error is not hereditary, or passed down from parent to child. Research suggests that women who have pregnancies after age 35 might be slightly more likely to have a son with KS. However, the extra X chromosome in KS comes from the father about half the time.3 

Citations

  1. Nielsen, J., & Wohlert, M. (1991). Chromosome abnormalities found among 34,910 newborn children: Results from a 13-year incidence study in Arhus, Denmark. Human Genetics, 87(1), 81–83.
  2. Klinefelter, H.F., Reifenstein, E.C., & Albright, F. (1942). Syndrome characterized by gynecomastia aspermatogenesis without A-Leydigism and increased excretion of follicle stimulating hormone. Journal of Clinical Endocrinology & Metabolism, 2, 615–627.
  3. National Human Genome Research Institute. Learning about Klinefelter syndrome. Retrieved June 5, 2012, from https://www.genome.gov/Genetic-Disorders/Klinefelter-Syndrome
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