Black F L, Hedrick P W
Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, CT 06520, USA.
Proc Natl Acad Sci U S A. 1997 Nov 11;94(23):12452-6. doi: 10.1073/pnas.94.23.12452.
The genotypic proportions for major histocompatibility complex loci, HLA-A and HLA-B, of progeny in families in 23 South Amerindian tribes in which segregation for homozygotes and heterozygotes could occur are examined. Overall, there is a large deficiency of homozygotes compared with Mendelian expectations (for HLA-A, 114 observed and 155.50 expected and for HLA-B 110 observed and 144.75 expected), consistent with strong balancing selection favoring heterozygotes. There is no evidence that these deficiencies were associated with particular alleles or with the age of the individuals sampled. When these families were divided into four mating types, there was strong selection against homozygotes, averaging 0.462 for three of the mating types over the two loci. For the other mating type in which the female parent is homozygous and shares one allele with the heterozygous male parent, there was no evidence of selection against homozygotes. A theoretical model incorporating these findings surprisingly does not result in a stable polymorphism for two alleles but does result in an excess of heterozygotes and a minimum fitness at intermediate allele frequencies. However, for more than two alleles, balancing selection does occur and the model approaches the qualities of the symmetrical heterozygote advantage model as the number of alleles increases.
对23个南美印第安部落家庭中后代主要组织相容性复合体基因座HLA - A和HLA - B的基因型比例进行了研究，这些家庭中纯合子和杂合子可能会发生分离。总体而言，与孟德尔预期相比，纯合子大量缺失（对于HLA - A，观察到114个，预期为155.50个；对于HLA - B，观察到110个，预期为144.75个），这与有利于杂合子的强烈平衡选择一致。没有证据表明这些缺失与特定等位基因或所采样个体的年龄有关。当这些家庭被分为四种交配类型时，对纯合子有强烈选择，在两个基因座上，三种交配类型的平均选择系数为0.462。对于另一种交配类型，即母本为纯合子且与杂合子父本共享一个等位基因的情况，没有证据表明对纯合子有选择。一个纳入这些发现的理论模型令人惊讶地没有导致两个等位基因的稳定多态性，但确实导致了杂合子过剩以及在中间等位基因频率下的最低适应度。然而，对于两个以上的等位基因，确实会发生平衡选择，并且随着等位基因数量的增加，该模型接近对称杂合子优势模型的特征。
