Dictionary of Human Evolution and Biology

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Crossing Over

Exchange of genetic material by breakage and rejoining between synapsed homologous chromosomes during prophase of meiosis I; in certain diploid organisms, events during meiosis leading to reciprocal exchange between homologous linkage groups (chromatids), resulting in recombination; structurally, the synapsis and chiasma of the arms of paired chromatids which have been contributed by a mother and a father; but see sister chromatid exchange. In some species only one sex exhibits crossing over (e.g. females in Drosophila); in humans, the recombination rate is higher in females than in males. Occurs very rarely during mitosis, as well.

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