Effect of population associations and reduced penetrance on observed and expected genotype frequencies in a simple genetic model: application to HLA and insulin dependent diabetes mellitus

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Abstract
Summary: The observed and Hardy‐Weinberg‐expected frequencies for (HLA) marker heterozygotes in a disease population are calculated where it is assumed that the disease is caused by either homozygosity or heterozygosity (with reduced pentrance) for a disease susceptibility allele at a disease locus, that allele being positively associated with both of the relevant alleles at the marker locus (the single susceptibility allele model of Svejgaard & Ryder, 1981). It is shown that the observed frequency is always less than or equal to the H‐W expectation, with the (observed/expected) ratio decreasing as the degree of dominance increases.
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