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Although human meiosis is regulated, errors, like nondisjunction, arise that yield aneuploid cells with absent or extra chromosomes.

Many nondisjunction incidents happen during meiosis I. Although the mechanisms remain unclear, this may result from mutations affecting how homologous chromosomes are initially joined, such as those disrupting the synaptonemal complex. Defects in the spindle apparatus can also be a contributing factor.

During anaphase I, homologous sets normally separate, or disjoin, and are dragged towards opposite poles. However, rarely, a chromosome pair fails to detach, and both are pulled to one end.

When meiosis ends, this type of nondisjunction may yield two cells that possess an additional chromosome and two that lack this structure.

Similarly, during meiosis II, sister chromatids may remain affixed during anaphase II. This may also result from spindle problems, or from cohesion defects, like how the centromere attaches chromatids.

As division finishes, such errors in meiosis II can produce two normal haploid cells. However, one cell with an extra and one with a lost chromosome are also generated.