<P> The term sex chromosome aneuploidy summarizes conditions with an abnormal number of sex chromosomes, i.e. other than XX (female) or XY (male). Formally, X chromosome monosomy (Turner syndrome, see above) can also be classified as a form of sex chromosome aneuploidy . </P> <P> Klinefelter syndrome is the most common sex chromosome aneuploidy in humans . It represents the most frequent cause of hypogonadism and infertility in men . Most cases are caused by nondisjunction errors in paternal meiosis I . About eighty percent of individuals with this syndrome have one extra X chromosome resulting in the karyotype XXY . The remaining cases have either multiple additional sex chromosomes (48, XXXY; 48, XXYY; 49, XXXXY), mosaicism (46, XY / 47, XXY), or structural chromosome abnormalities . </P> <P> The incidence of XYY syndrome is approximately 1 in 800 - 1000 male births . Many cases remain undiagnosed because of their normal appearance and fertility, and the absence of severe symptoms . The extra Y chromosome is usually a result of nondisjunction during paternal meiosis II . </P> <P> Trisomy X is a form of sex chromosome aneuploidy where females have three instead of two X chromosomes . Most patients are only mildly affected by neuropsychological and physical symptoms . Studies examining the origin of the extra X chromosome observed that about 58 - 63% of cases were caused by nondisjunction in maternal meiosis I, 16 - 18% by nondisjunction in maternal meiosis II, and the remaining cases by post-zygotic, i.e. mitotic, nondisjunction . </P>

What is a chromosomal disorder that can result from nondisjunction