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What is the expected amount of DNA that is shared by all of the people between first cousins, including the first cousins, in a family tree?

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To simplify your problem, let's just consider autosomal DNA. In this case, your expected amount of shared DNA should be synonymous with genetic consanguinity, which can be conveniently checked from this image. enter image description here

Thus, you share approximately 1/8 of DNA(I know saying so is not scientifically strict as there are recombinations and mutations that prevent the DNA to be exactly the same.) with your first cousins. Same is true between first cousins.

Source: Wikipedia image gallery.

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  • $\begingroup$ How much of the 12.5% shared DNA between first cousins is also shared by, say, their parents who are siblings? Is it a lower expected percentage which is common to all four people? $\endgroup$
    – user39121
    Commented Dec 30, 2017 at 15:21
  • $\begingroup$ @michaelc35 Their parents who are siblings should share more DNA in common, 25% in fact. You can think it in this way. Your first cousins' DNA is diluted by other people who married into your family, each of your first cousins only carry a half of DNA from their parents who are siblings, so they just have 12.5% in common. $\endgroup$
    – Macrophage
    Commented Dec 30, 2017 at 15:31
  • $\begingroup$ @michaelc35 If you are satisfied with the answer, please accept it manually. It will encourage me to produce more quality answers in the future, thanks! :D $\endgroup$
    – Macrophage
    Commented Dec 30, 2017 at 15:32
  • $\begingroup$ I'm interested in what the expected amount of DNA that all four people have in common - the two first cousins and their parents who are siblings. It seems that the expected percentage of DNA shared by all four would be lower than 12.5%. $\endgroup$
    – user39121
    Commented Dec 30, 2017 at 15:35
  • $\begingroup$ @michaelc35 Yes, definitely it would be lower. But there is no precise value in this case. An averaged estimate will put it at a half of 12.5% considering the amount of DNA shared between a first cousin and the sibling of his/her parents. $\endgroup$
    – Macrophage
    Commented Dec 30, 2017 at 15:42

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