Final answer:
Nondisjunction during maternal meiosis I is the most common cause of Down syndrome, resulting in an extra chromosome 21 in the gametes. This risk increases with maternal age, especially in women over 36.
Step-by-step explanation:
The most common meiotic error resulting in Down syndrome is nondisjunction during maternal meiosis I. This form of nondisjunction occurs when the homologous chromosomes fail to separate, leading to an extra chromosome 21 in the resultant gametes. It is known that this type of error is more likely to occur as the mother's age increases, particularly in women over 36 years old. Notably, the age of the father is considered less significant in the frequency of these nondisjunction events since they occur less frequently in sperm.
The error of nondisjunction can result in various gametes: if occurring in meiosis I, two gametes will have an extra chromosome (n + 1) and two will be missing one (n - 1). If it occurs during meiosis II, it results in one gamete with an extra chromosome (n + 1), one missing a chromosome (n - 1), and two standard gametes (n). As a result, nondisjunction during maternal meiosis I is most often responsible for Down syndrome.