Numeric sex chromosome variations
Type of disease: Rare conditions
Numeric sex chromosome variations refers to differences in the number of sex chromosomes that may be present in an individual’s body cells. Males typically have one X chromosome and one Y chromosome in each body cell with a total of 46 chromosomes (46, XY); females typically have two X chromosomes in each body cell (46, XX). Numeric sex chromosome variations include chromosome findings with extra or missing X or Y chromosomes. Whether there are associated signs or symptoms, as well as severity of features, can vary greatly depending on the variation present. The most common numeric sex chromosome variations include Turner syndrome (45, X); 47, XXY (which usually causes features of Klinefelter syndrome); 47, XYY; and 47, XXX (trisomy X). Most numeric sex chromosome variations are not inherited and occur due to a random error during the formation of reproductive cells (eggs and sperm) in a parent. In some cases, individuals have some cells with the usual 2 sex chromosomes, and some cells with extra or missing sex chromosomes; this is called chromosomal mosaicism.
Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.