As it is depicted in most textbooks, cross-over does not occur between the two "outer" sister chromatids. By independent assortment during Meiosis I, there is 1/2^23 chance that all father's chromosomes, with one pure father's chromatid, will end up in the same daughter cell. Does this mean that roughly one in eight million people does not have any genetic material from one of his grandparents or is there some more independent assortment during Meiosis II, which makes this outcome extremely unlikely? (more like 1/2^46)

More generally I would be interested in the probability that a gamete will contain more than 60% ,70%, 80% ... of only one of your parent's genes.

  • $\begingroup$ Welcome to Biology.SE! Please take the tour and then go through the help pages starting with How to Ask questions effectively on this site and edit your question accordingly. In particular, it would be helpful if you could be clearer about what you are asking — for example, it seems that you sometimes use parent when you really mean grandparent. Thanks! 😊 $\endgroup$ – tyersome Jan 19 at 17:16
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    $\begingroup$ In organisms like humans it is VERY rare for sister chromatids not to undergo at least once crossover. You may want to look into the term obligate crossover. It's not entirely clear from your question, but your reference to 'two "outer" sister chromatids" makes me wonder if you are confusing a convenience in drawing an illustration with a biological fact. $\endgroup$ – Charles E. Grant Jan 19 at 17:56
  • $\begingroup$ So if I understand it correctly, it is very unlikely that any of the four chromatids in the tetrad will remain unchanged after the crossover occurs, right? $\endgroup$ – Flitwick Jan 19 at 21:04

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