How Genes can be exchanged between chromatids?

How Genes can be exchanged between chromatids? 
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When two homologous chromosomes physically exchange corresponding segments during prophase I of meiosis, geneticists call it crossing over. Recombinations occur at chiasmata during pachytene of meiosis-I. If just a few exchanges occur, genes that are closer together tend to stay together. The farther apart on the same chromosome genes are, the more likely they will separate during recombination. The two extremes are independent assortment and complete or absolute linkage. The progeny resulting from crossing over appear in repeatable proportions, called the recombinant frequency. Greater recombination frequencies are observed for genes that are farther apart on the chromosomes because a chiasma is more likely to cut between genes that are far apart than genes that are closer together.  
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