MadSci Network: Genetics

Re: Who is most genetically similar siblings or double first cousins?

Date: Fri May 10 08:06:12 2002
Posted By: Jeremy Cherfas, Staff, Public Awareness, IPGRI (International Plant Genetic Resources Institute)
Area of science: Genetics
ID: 1019845303.Ge

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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