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I have been trying to derive the coefficient of inbreeding lately, but I am at a loss. If I understand correctly it measures the probability of an allele "collision" i.e. getting the same allele from both father and mother.

As an urn problem offspring from two sibling can be stated as having two urns with marbles $\{a, b, c, d\}$ in one and $\{a, e, c, g\}$ in the other, so the intersection between the two are $\{a, c\}$. In each draw you take one marble from each urn without replacement. What is the probability of drawing at least one "collision" over 4 draws, i.e. a situation where either $(a,a)$ or $(c,c)$ comes up? What about the probability of having two collisions?

I have an idea that it is related to the hypergeometric distribution, but I can't figure out how to include that drawing $(a,c)$ removes the possibility of of drawing $(c,c)$.

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    $\begingroup$ You can start by noticing that this is an extension of birthday paradox problem. $\endgroup$ – Tim Nov 21 '16 at 13:14

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