Chunk #14 — Mathematical Details — Correlation coefficient of two relatives’ allele counts

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Effective sample size: Quick estimation of the effect of related samples in genetic case-control association analyses.

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The correlation coefficient between allele count x (x=2,1,0 for marker genotype AA, AB, BB, with probability of p2, 2pq, q2) of two sibs is: r≡Cov[xsib1,xsib2]Var[xsib1]Var[xsib2]=E[xsib1,xsib2]−E[x]2Var[x].