Trisomy 21 Genetics
Three genetic variations can cause Down syndrome. In most cases (approximately 92 percent of the time), Down syndrome is caused by the presence of an extra chromosome 21 in all cells of the individual. Since three copies of chromosome 21 are present, the condition is called trisomy 21.
In such cases, the extra chromosome originates in the development of either the egg or the sperm. Consequently, when the egg and sperm unite to form the fertilized egg, three -- rather than two -- chromosomes 21 are present. As the embryo develops, the extra chromosome is repeated in every cell.