Nondisjunction is the failure of chromosomes or chromatids to separate in meiosis.

This can occur in meiosis I when homologous pairs 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: