What does nondisjunction refer to in meiosis?

Nondisjunction refers specifically to the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. This can occur in either the first or second meiotic division. When nondisjunction happens, it leads to gametes that have an abnormal number of chromosomes, which can result in conditions like Down syndrome or Turner syndrome when these gametes participate in fertilization and form a zygote.

In the context of meiosis, proper separation of chromosomes is crucial for ensuring that each gamete receives the correct number of chromosomes. The other factors mentioned, such as excessive chromosome replication and reduction in chromosome number, do not accurately describe nondisjunction. Instead, they pertain to different aspects of chromosomal behavior and genetic variation, making the understanding of nondisjunction vital for studying genetic disorders and meiosis.