What fails to separate during non-disjunction in anaphase 1 of meiosis?

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During anaphase 1 of meiosis, the key event is the separation of homologous chromosomes, which are pairs of chromosomes, one inherited from each parent. Non-disjunction occurs when these homologous chromosomes fail to separate properly. As a result, one daughter cell receives both copies of a particular chromosome, while the other daughter cell receives none. This can lead to gametes with an abnormal number of chromosomes, which can result in various genetic disorders upon fertilization.

Sister chromatids, which are identical copies formed after DNA replication, are still held together by centromeres during anaphase 1 and only separate during anaphase 2 of meiosis. Therefore, it is the failure of homologous chromosomes to separate during anaphase 1 that characterizes non-disjunction in this phase, not the other processes or structures involved in meiosis.