Non-disjunction is the failure of chromosomes to separate during either mitosis, meiosis I or meiosis II. The consequence of non-disjunction is aneuploidy: an imbalance in the number of chromosomes.

In meiosis I, the consequence of non-disjunction is that 50% of gametes have one additional homologous chromosome (trisomy), while 50% have one fewer homologous chromosome (monosomy). In meiosis II, the non-disjunction only affects the 50% of gametes where the faulty division occurred: one of these gametes will be monosomic and the other trisomic.