Final answer:
Meiotic nondisjunction is an error during meiosis where chromosomes fail to separate, resulting in gametes with too few or too many chromosomes. This can occur during either meiosis I or II, leading to varying distributions of chromosomal numbers in the resulting gametes and potentially causing disorders such as Down syndrome.
Step-by-step explanation:
The term meiotic nondisjunction refers to the failure of chromosomes to separate normally during meiosis, which can lead to gametes with abnormal numbers of chromosomes. If nondisjunction occurs during meiosis I, it results in two gametes that are missing a chromosome (n - 1) and two gametes with an extra chromosome (n + 1). When nondisjunction happens during meiosis II, one of the resulting gametes will lack a chromosome (n - 1), one will have an extra chromosome (n + 1), and the other two will have the expected number of chromosomes (n). This can lead to disorders such as Down syndrome, which is characterized by an extra copy of chromosome 21, also known as trisomy 21.