At least 1 out of 500 persons has an extra X and/or Y chromosome**

An estimated 1 out of 900 females are Trisomy X, and 1 out of 900 males are XYY.  Together with the estimated 1 out of 600 males who are XXY, these and associated X and/or Y chromosome variations affect more than 1 out of every 500 persons.

Source:  Bojesen et al., “Pre-Natal and Post-Natal Prevalence of Klinefelter Syndrome,” Journal of Clinical Endocrinology & Metabolism (2003).