Researchers estimate that 1 male in about 500 newborn males has an extra X chromosome, making KS among the most common chromosomal disorders seen in all newborns.1 The likelihood of a third or fourth X is much rarer: 2
Prevalence of Klinefelter syndrome variants
|Number of extra X chromosomes||One (XXY)||Two (XXXY)||Three (XXXXY)|
|Number of newborn males with the condition||1 in 500||1 in 50,000||1 in 85,000 to 100,000|
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 (pronounced huh-RED-i-ter-ee) or passed down from parent to child. Research suggests that older mothers might be slightly more likely to have a son with KS. However, the extra X chromosome in KS comes from the father about one-half of the time.3
- 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.
- 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.
- National Human Genome Research Institute. Learning about Klinefelter Syndrome. Retrieved on June 5, 2012 from http://www.genome.gov/19519068