What is nondisjunction in the context of meiosis?

Nondisjunction specifically refers to the failure of chromosomes to separate properly during meiosis. This error can occur during either the first or second meiotic division. In meiosis I, homologous chromosomes may fail to separate, leading to gametes with an abnormal number of chromosomes. In meiosis II, the failure can occur among sister chromatids. The result is that some gametes receive an extra chromosome (n+1) while others may end up with one less (n-1). When fertilization occurs, this abnormal gamete distribution can lead to conditions such as Down syndrome, which is caused by the presence of an extra copy of chromosome 21.

The other options pertain to various aspects of meiosis but do not accurately describe nondisjunction. The separation of chromatids into different cells refers to a normal process that occurs during either meiotic division. The duplication of all chromosomes describes the replication step prior to meiosis but does not relate to improper separation. The addition of extra chromosomes lacks specificity and isn’t a standalone process in meiosis; it is a possible result of nondisjunction, but not the definition itself.