Evidence for balancing of KM, but not GM, alleles by heterotic advantage in South Amerinds.

Two KM alleles occur in 1075 South Amerinds of 14 tribes in approximately balanced and uniform frequency. However, the number of heterozygotes is 12.7% greater than expected by frequency analysis and 16.5% greater by segregation analysis. This excess is evident in children 0-4 years of age and may reflect either prenatal or early childhood selection. The frequencies of GM haplotypes were different, and quite uniformly so, in diverse tribes. Most GM heterozygotes can only be distinguished from GM 1,2,17 21 homozygotes by DNA or family relationship. No deficit of GM homozygotes was observed in 119 children in whom heterozygosis was determined by family. Thus, the KM polymorphism, like HLA, may be maintained by preferential survival of heterozygotes, but GM probably depends on another mechanism.