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I understand the mechanics of recombination, but am struggling with the 'scale'.

  1. When two homologous chromosomes pair, roughly how many recombination events occur on average? I understand there will be a distribution, and it will vary between species - but are we talking one, two, or hundreds/thousands?

  2. How are long are recombination fragments relative to the entire length of the chromosome? With the same caveat as above, I'm trying to get a sense of scale - 0.001%, 1%, 10%, 25%?

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When crossover happens, it's a whole swap from that point on, not just a little chunk. So if there's only one crossover event, then the lengths are "p" and "100% - p", where p is the position along the chromosome. The only way to get a small piece of one chromosome amidst a mostly intact copy of the other would be for there to be two crossovers close to each other.

This existing Q&A talks about the rate of crossovers in human chromosomes:

How many recombination events are there per generation in humans?

It varies by which chromosome you're talking about and other factors, but something along the order of 1 to 4 events per chromosome would be a reasonable ballpark. 0 is possible, too.

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