<P> Human trisomies compatible with live birth, other than Down syndrome (trisomy 21), are Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13). Complete trisomies of other chromosomes are usually not viable and represent a relatively frequent cause of miscarriage . Only in rare cases of a mosaicism, the presence of a normal cell line, in addition to the trisomic cell line, may support the development of a viable trisomy of the other chromosomes . </P> <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>

Nondisjunction of all three chromosome pairs in meiosis i