Final answer:
Nondisjunction is the failure of chromosomes to separate during meiosis I or II, leading to the production of gametes with abnormal numbers of chromosomes, causing conditions like Down syndrome.
Step-by-step explanation:
Nondisjunction is the occasional failure of 2 chromosomes to separate from each other during meiosis, a crucial process in the formation of gametes.
This event can occur in two different phases of meiosis. During meiosis I, if homologous chromosomes fail to separate, this can result in two gametes lacking that particular chromosome and two gametes with an extra copy.
Alternatively, if nondisjunction happens during meiosis II, when sister chromatids fail to separate, it can lead to one gamete with a missing chromosome, two normal gametes with one chromosome copy, and one gamete with an extra copy.
The consequences of nondisjunction can be significant, resulting in aneuploidy, a condition where the gametes have an abnormal number of chromosomes, potentially leading to genetic disorders such as Down syndrome, Turner syndrome, and Klinefelter syndrome.