In nondisjunction, if members of a pair of homologous chromosomes do not move apart during meiosis I or sister chromatids fail to separate during meiosis II, this means one gamete receives 2 of the same kind of chromosome and if either one of these abnormal gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of particular chromosomes; which is known as an aneuploidy.
