Down’s Syndrome is caused by (AIIMS May 2014)
a. Nondisjunction occurs when a homologous pair of chromosomes fails to disjoin at the first meiotic division, or the two chromatids fail to separate either at the second meiotic division or in somatic cell divisions, resulting in two aneuploid cells.
b. When nondisjunction occurs during gametogenesis, the gametes formed have either an extra chromosome (n+l) or one less chromosome (n-1). Fertilization of such gametes by normal gametes results in two types of zygotes—trisomic (2n+l) or monosomic (2n-1).