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