What refers to the failure of sister chromatids to separate during mitosis or meiosis?

Nondisjunction refers to the event in which sister chromatids fail to separate properly during cell division, either in mitosis or meiosis. This can lead to gametes with an abnormal number of chromosomes, potentially resulting in conditions such as Down syndrome when an abnormal egg or sperm fuses with a normal one. It's a critical error in the process of chromosome segregation, which should normally ensure that each daughter cell receives a complete set of chromosomes.

In contrast, crossing over involves the exchange of genetic material between homologous chromosomes during meiosis, while segregation refers to the normal process of separating alleles during gamete formation. Genetic recombination is a broader term that encompasses the mixing of genetic material that can occur due to processes like crossing over. Therefore, nondisjunction is specifically about the failure of proper chromosomal separation, distinguishing it from these other cellular processes.