Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. If non-disjunction occurs during meiosis I, the homologous chromosomes fail to separate. This means that one secondary oocyte will receive both homologous chromosomes, and the other will receive none. After meiosis II, the resulting four gametes will consist of two gametes with n+1 chromosomes (due to the extra chromosome) and two gametes with n-1 chromosomes (due to the missing chromosome). In contrast, non-disjunction in meiosis II would result in two normal gametes (n), one gamete with n+1, and one gamete with n-1 chromosomes.
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