Final answer:
Nondisjunction can occur during meiosis I or II. Failure of homologous chromosomes to separate during meiosis I results in gametes with chromosomal imbalances. Failure of sister chromatids to separate during meiosis II also leads to chromosomal imbalances in resulting gametes.
Step-by-step explanation:
Nondisjunction can occur during either meiosis I or II. If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that chromosome and two gametes with two copies of the chromosome. If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.