What does nondisjunction refer to in meiosis?

Nondisjunction during meiosis specifically refers to the failure of homologous chromosomes to separate during the first meiotic division (meiosis I) or the failure of sister chromatids to separate during the second meiotic division (meiosis II). When this separation does not occur, it results in gametes that contain an abnormal number of chromosomes.

Choosing homologous chromosomes is crucial because their improper separation can lead to conditions such as Down syndrome, which is caused by an extra copy of chromosome 21. The significance of nondisjunction in meiosis lies in its potential to create aneuploidy in offspring, where the resulting cells have either too many or too few chromosomes, leading to genetic disorders or developmental issues.

The other options describe different phenomena related to reproductive biology. The fertilization of the ovum is unrelated to chromosomal separation during meiosis, and the failure to produce haploid cells signifies a failure of the whole meiotic process rather than a specific event of nondisjunction. Similarly, the separation of sister chromatids is a different process that occurs in meiosis II and while it can also lead to nondisjunction, the terminology specifically in the context of the question is more appropriately focused on homologous chromosomes.