Effect of inbreeding on IQ and mental retardation.

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Abstract
Both decline of IQ and increase of mental retardation are consistent with rare recessive alleles at about 325 loci. There is no suggestion of a discrepancy that might be due to polygenic dominance or confounding of cosanguinity with unfavorable environment. The risk for mental retardation in matings of normal parents increases from 0.012 with random mating to 0.062 for 1st-cousin parentage, but dominance deviations are a negligible cause of family resemblance of IQ. Implications for gene frequencies, mutation rates and radiation response are detailed.