Nondisjunction
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Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leaving gametes with an abnormal chromosome number.
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A homolog pair that fails to separate at anaphase I sends both copies into one gamete and none into the other.
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Nondisjunction can occur in meiosis I or meiosis II, and the two failures produce different mixes of abnormal gametes.