Genetic Structure of the HL‐A System in a Nahua Indian Population in Mexico

HL–A phenotype and gene frequencies are presented for a population sample of Nahua Indians from Mexico. These Indians showed restricted variability in the first and second segregant HL–A series as compared to Caucasian, Black and Mongoloid populations, thus resembling other American Indian populations. Determinants for HL–A2, HL–A9, W28 and W31 accounted for 95% of first series alleles. For the second series W16, W5 and W10 comprised 75% of all genetic determinants. This population sample is the only American Indian group so far studied with W16 as the most frequently occuring second series antigen. Disturbances in the Hardy‐Weinberg equilibrium for the second series may reflect racial admisture and an antigen which cross reacts with W5 and W15. When adjustments were made for sample size and selection of the extreme deviate delta value, no evidence of linkage disequilibrium was noted in this population. Thus far, all American Indian groups studied show restricted variability of the HL–A system; this may reflect the effects of natural selection.