The degree to which the genotype frequencies at each locus agree with Hardy–Weinberg equilibrium (HWE) expectations is also shown in Table 1. This was determined at the regional level, and not for individual populations, as many of the individual samples were too small to allow the agreement with HWE to be calculated accurately. Agreement with HWE (P > 0.05) was seen in 16 out of 18 comparisons. In some cases, the expected values obtained for some genotype frequencies were so low as to introduce bias into the chi-square calculation , which may explain the disagreement seen at CCR5-Δ32 in Asian populations. The other exception (CCR5-59653T in the Americas) was due to an observed excess of homozygotes and a deficit of heterozygotes. The reason for this is not clear, but instead of reflecting an aspect of population structure, this could be a Type I error; a probability threshold of 0.05 would result in HWE being falsely rejected in one out of every 20 tests performed. We conclude that there is little or no selective or other effect of CCR2-64I or CCR5-59653T that is sufficient to perturb the genotype frequencies.
The authors thank the many researchers who provided the samples for our original study of CCR5-Δ32, and to C. Christodoulou and A. Hatzakis for additional samples.
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