Figure 1.

Population heterozygote advantage as a function of allele-specific effects and allele frequencies. Parameter regions in which heterozygotes will on average have a higher probability of a favorable disease outcome than homozygotes (regions of population heterozygote advantage) are shown in black. Population heterozygote advantage occurs when diversity of resistant alleles is sufficiently high and diversity of susceptible alleles is sufficiently low i.e., toward the bottom right of the parameter space in each panel of the figure. Different panels indicate various assumptions about the genotype-specific relative risks a-d (defined in Table 1). Parameters: Overdominant (a = 1.1, b = 1.6, c = 2, d = 1.5); dominant (a = 1, b = c = d); additive (a = 1, b = (1 + c)/2, c = d); recessive (a = b = 1, c = d); underdominant (a = 0.5, b = 0.9, c = 1.4, d = 1.5). These curves are drawn for p = 0.5. Dominant, additive and recessive curves are valid for all possible values of the free parameters, while underdominant and overdominant curves are examples whose positions depend on the particular values of the parameters a, b, c and d.