Final answer:
Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, leading to an abnormal chromosome number. This can cause aneuploidy, resulting in conditions like Down syndrome and Turner syndrome.
Step-by-step explanation:
Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number. This can lead to aneuploidy, which is a condition where the zygote has too many or too few chromosomes. For example, if nondisjunction occurs during meiosis I and a pair of homologous chromosomes fail to separate, one gamete will have an extra chromosome and the other gamete will be missing a chromosome. This can result in numerical abnormalities such as Down syndrome (trisomy 21) or Turner syndrome (monosomy X).