In which phase of meiosis do homologous chromosomes fail to separate if nondisjunction occurs?

Nondisjunction refers to the failure of homologous chromosomes or sister chromatids to separate properly during cell division, which can lead to gametes with an abnormal number of chromosomes. This specific scenario occurs during meiosis, which consists of two stages: meiosis I and meiosis II.

In meiosis I, homologous chromosomes line up and pair at the metaphase plate during metaphase I. During this phase, if nondisjunction occurs, one pair of homologous chromosomes fails to separate and moves together to one pole of the cell instead of being distributed to opposite poles. This results in one daughter cell receiving both chromosomes of the homologous pair, while the other daughter cell receives none.

This is crucial because it sets the stage for further chromosomal imbalances in the resulting gametes and, ultimately, in any offspring produced from those gametes. In contrast, nondisjunction can also occur during meiosis II when sister chromatids fail to separate, but in this case, it would not involve homologous chromosomes but rather the sister chromatids of a single chromosome.

Thus, the occurrence of nondisjunction with homologous chromosomes specifically happens during metaphase I, making it the correct answer.