, Down's syndrome is caused by the presence of an additional copy of chromosome number 21 (trisomy of 21). This aneuploidy results from the failure of segregation of chromatids during the cell division cycle (meiosis) results in the gain or loss of a chromosome. This failure of segregation, known as non-disjunction, can occur during the formation of either the egg or the sperm, leading to a gamete with an extra chromosome , .