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In Meiosis I, 46 chromosomes crossover. These mixed chromosomes then split off into new cells. In these new cells, Meiosis II occurs, where these mixed chromosomes have their chromatids ripped apart.

What I'm confused about is why does Meiosis II produce 2 different new cells if you're just separating 2 identical chromatids? Is it because when crossing over the genes makes the both chromatids different?

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  • $\begingroup$ would someone care to explain why the downvote? I think this question meets community guidelines $\endgroup$ – John Hon May 22 '20 at 4:57
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Indeed, this is all due to the crossing over between different alleles. For example, put the case of one cell that contains 2 chromosomes, one from the mother and another from the father. Each chromosome contains two alleles: Ab (mother) and aB (father).

The first thing that they will do is divide into two chromatids each one getting to: Ab[1M] (one mother's chromatid) and Ab[2M] (second mother's chromatid) and the same for the father aB[1F] and aB[2F]. Notice that the chromatids still joined by the centromere.

Now, the crossing over comes: in this case, if the A[2M] from mother's chromosome gets exchange with a[1F] of the father's chromosome, we get: Ab[1M], ab[2M], AB[1F], aB[2F].

As you see, when they segregate you will get 4 different cells with two alleles: Ab, ab, AB and aB.

I hope this answers your question.

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