|MadSci Network: Genetics|
Dear Sandra This is a slightly tricky question, but I hope you will be able to follow my argument. I'm not exactly sure I know what you mean by double first cousins. I think you mean that A and B are sibs, and C and D are sibs from an unrelated family, and A has offspring with C while B has offspring with D. Sibs, as I am sure you know, are related by 0.5. That is, there is a 0.5 probability of a particular gene being present (by descent) in two sibs that share the same mother and father. If E and F are double first cousins, the first thing to state is that they cannot be inbred. That is, they do not share any copies of the same gene (more correctly the same allele) that are identical by descent. That means we can use the same approach to relatedness as we do in the case of sibs. Take a random gene in E, and ask what is the probability that F has a copy of it? The answer is that it came from one parent (doesn't matter which). There is thus half a chance that it is also in that parent's sib, and then half a chance it was passed on to the parent's sib's offspring, the double first cousin. Thus the relatedness of double first cousins is 0.25. Another way to see this, which may be easier, is that in the absence of inbreeding, the relatedness accrued through different paths adds up. First cousins have r=1/8. Thus double first cousins (with two separate paths for gene connections) must have twice that relatedness. Best wishes Jeremy Cherfas
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