How does meiosis ensure genetic diverlty

Two ways: 
1. In prophase I of meiosis the homologous chromosomes synapse and crossing over occurs. This switches sections of two of the sister chromatids so the outer two chromatids have the same mix of alleles as the parents and the inner two chromatids have new combinations of alleles. 
2. Meiosis puts only one of each kind of chromosome in the gametes, selecting one of each homologous pair at random. Then when each gamete joins with the gamete from the other parent, there are practically infinite combinations of the alleles possible.

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