Final answer:
Nondisjunction is the failure of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during meiosis II, leading to the formation of aneuploid gametes.
Step-by-step explanation:
Nondisjunction is the failure of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during meiosis II, leading to the formation of aneuploid gametes. Aneuploidy is a condition in which there is an abnormal number of chromosomes in the gametes, resulting in disorders such as Down syndrome or Turner syndrome. Nondisjunction can occur in either meiosis I or II, and the specific outcome depends on when and which chromosomes fail to separate.