Final answer:
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to gametes with an abnormal number of chromosomes, which can cause various abnormalities in the offspring.
Step-by-step explanation:
An egg or sperm with too many or too few chromosomes is a result of a phenomenon known as nondisjunction. During meiosis, if homologous chromosomes fail to separate in meiosis I, or if sister chromatids fail to separate during meiosis II, this leads to the formation of aneuploid gametes, which are gametes with an abnormal number of chromosomes (n+1, n-1, or n).
Problems like physical and mental abnormalities may arise from this genetic imbalance in the resulting offspring, potentially manifesting as a range of disorders, depending on which chromosomes are involved. Nondisjunction can occur at two distinct points in the cell division process: either during meiosis I, resulting in all resulting gametes being affected (each with either n+1 or n-1 chromosomes), or during meiosis II, resulting in a mixture of affected and unaffected gametes.
When comparing nondisjunction in meiosis I and II, if nondisjunction occurs in meiosis II, typically 50% of the gametes are normal and 50% are aneuploid, whereas nondisjunction during meiosis I often results in all gametes being aneuploid.