What is nondisjunction during meiosis?

Nondisjunction during meiosis refers to the failure of homologous chromosomes or sister chromatids to separate properly during the division process. This abnormal separation can occur during either meiosis I or meiosis II, resulting in gametes that have an abnormal number of chromosomes. When fertilization occurs, these gametes can lead to zygotes with trisomy or other chromosomal anomalies, depending on whether there is an extra chromosome or a missing chromosome.

Understanding nondisjunction is critical in genetics, as it explains many conditions that arise due to chromosomal abnormalities, such as Down syndrome, which is caused by an extra copy of chromosome 21. In contrast, options that suggest normal chromosomal separation or the creation of gametes with a normal chromosome count do not represent the concept of nondisjunction and its implications. The exchange of genetic material between chromosomes is a separate process known as crossing over and does not pertain to the concept of nondisjunction.