Nondisjunction is a type of chromosomal mutation that occurs when chromosomes fail to separate properly during cell division, resulting in an unequal distribution of chromosomes into daughter cells. As a result, some cells end up with an extra chromosome, while others are missing a chromosome.
Nondisjunction can occur in both meiosis and mitosis. In meiosis, nondisjunction can occur during the first division (meiosis I) or the second division (meiosis II). If nondisjunction occurs during meiosis I, the resulting gametes will have an extra or missing chromosome. For example, if a pair of homologous chromosomes fails to separate during meiosis I, one gamete will receive both copies of the chromosome, while the other gamete will receive none. If nondisjunction occurs during meiosis II, the resulting gametes will have an extra or missing chromatid.
In mitosis, nondisjunction can occur during the separation of sister chromatids, resulting in one daughter cell having an extra chromosome and the other having a missing chromosome.
Nondisjunction can lead to genetic disorders such as Down syndrome, Turner syndrome, and Klinefelter syndrome. These disorders are caused by an abnormal number of chromosomes in the affected individual's cells.