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Crossing over is the mutual exchange of corresponding segments of non-sister chromatids of homologous chromosomes at the pachytene substage of meiosis I to produce new genetic recombinations. Because of crossing over only, recombinations can occur. The exchange of segments between the non-sister chromatids of two homologues is so exact that neither of the two gains or loses any gene. If they do not undergo crossing over, then no recombination will take place and so, much of the parental traits will be seen in the offsprings called as 'Linkage'. I will send you a mail on this.
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