Dictionary of Human Evolution and Biology

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Premeiotic Phenomena

During oögenesis, female mammals differ from males in that premeiotic phenomena take place in the late first trimester of a female fetus, whereas in males these events are postponed until puberty. These consist of the initial stages of meiosis through late prophase/early metaphase, such that the chromatids are synapsed and are chiasmate, at which point meiosis is suspended and not resumed until ovulation. This profoundly influences the probability of nondisjunction in females of advanced age; see maternal age effect.

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