Nondisjunction is the failure of chromosomes or chromatids to separate in meiosis.
This can occur in meiosis I when pairs of homologous chromosomes fail to separate during anaphase I.
Nondisjunction can also occur in meiosis II
when sister chromatids fail to separate during
anaphase II.
Both result in gametes with incorrect number of chromosomes, or aneuploidy.
Meiosis II nondisjunction animation:
