Nondisjunction is the unequal distribution of chromosomes during cell division.
This can occur in meiosis I when homologous chromosomes fail to separate during anaphase I.
Nondisjunction can also occur in meiosis II when sister chromatids fail to separate during anaphase II.
This produces gametes with incorrect number of chromosomes.
Most zygotes from such gametes are not viable; survivable cases include trisomy 21 and sex chromosome abnormalities.
Meiosis II nondisjunction animation:
