Why does multiple allels doesn't perform crossover??

Meiotic cell division has an event called crossing over. During this event, a short segment of DNA on one chromatid is exchanged with a length of DNA on the sister chromatid. Crossing over has no effect if the alleles are same but a reassortment of linked alleles occur if they are different, as exhibited by Mendelian principle of independent assortment. Now, we know that when there are more than two alleles determining a trait, we call them multiple alleles (for example blood group of human beings are determined by three alleles, namely IA, IB, and i). Remember that a cell can, at any time, hold two of these alleles. Moreover, all the different alleles for the same trait occupy the same locus. Thus, multiple alleles do not perform crossing over.

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