Final answer:
Nondisjunction refers to the improper segregation of chromosomes during meiosis I or II, which can lead to gametes with abnormal chromosome numbers and genetic disorders.
Step-by-step explanation:
The failure of chromosomes to segregate properly during mitosis or meiosis is called nondisjunction. Nondisjunction can occur if there is a failure of synapsed homologs to completely separate and migrate to opposite poles during the first cell division of meiosis, known as meiosis I, or if there is a failure of replicated chromosomes, or sister chromatids, to separate during meiosis II. This mishap can result in the formation of gametes with an abnormal number of chromosomes, potentially leading to disorders due to aneuploidy. These disorders include well-known conditions such as Down syndrome, which is the result of an extra copy of chromosome 21.