Talk about non-disjunction in meiosis I and II.

Aneuploid variations are originated during gametogenesis, phenomenon called non-disjunction, where homologous chromosomes fail to disjoin, giving either two members of the affected chromosomes or non at all. Fertilizing them with normal haploid germ cell gives either a trisomy or monosomy, leading to a variety of aneuploid conditions. Trisomy 21 frequently occurs during meiosis, during either anaphase I or II, about 75% during meiosis I.