12. d. nondisjunction
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Nondisjunction means the failure to separate. In genetics it means the failure of two homologous chromosomes to separate during meiosis. It can also mean the failure of sister chromatids to pass to separate cells in mitosis II ( or in mitosis). Nondisjunction results in aneuploidy--the condition in which body cells do not have the normal diploid number (2n) of chromosomes.