Down’s syndrome is an autosomal aneuploidy, caused by the presence of an extra chromosome number 21. Both the chromosomes of the pair 21 pass into a single egg due to non-disjunction during oogenesis. The non-disjunction is more common in females. The frequency of non- disjunction of chromosome pair 21 increases with mothers age. Usually non-disjunction of chromosomes takes place in meiosis I.