Crossing over occurs in Prophase I of Meiosis I, which occurs between non-winter chromatids of homologous chromosomes. Crossing over results in chiasma being formed, between homologous regions of non-sister chromatids, and synapsis occurs. At Anaphase I, homologous chromosomes separate, and the chiama are broken apart. This results in the previous identical sister chromatids of each chromosome to become non-identical, as the sister chromatid that paired up with the sister chromatid of its homologous chromosomes has a different combination of alleles, which gives rise to different combination of characteristics. Thus, variation occurs in the offspring that attains the sister chromatid (later known as daughter chromosome after Anaphase II).
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