Calculation of equilibrium allele frequency () under heterozygote advantage
Recall GSM: q = [pq] [(q)(W2 - W1) + (p)(W1 - W0)] /

genotype:    AA     AB        BB                                              
phenotype:  W0    W1     W2    (all phenotypes different)
or              1 - s1    1     1 - s2

At equilibrium where q = 0  , call q = (read as 'q hat'))
Neither [pq] or can be zero, so solve for middle term

q = (q)(W2 - W1) + (p)(W1 - W0)
0 = ()(1 - s2 - 1) + (p)(1 - 1 + s1)
   = - s2 + s1p
   = (1 - )(s1) - s2
   =  s1 - (s1 + s2)()
s1 = (s1 + s2)()

 = (s1) / (s1 + s2)
    The heterozygote advantage model is sometimes called "Overdominance". This is an unfortunate term, as 'dominance' is a relationship among alleles at a locus, but "overdominance" describe the fitness phenotype of the Aa genotype, relative to that of the AA and aa genotypes. The companion term "underdominance" to describe heterozygote disadvantage creates the same problem. Like use of  'gene' when we mean 'allele', the latter terms are too well established to discard outright, but should be avoided.
Figure © 2013 by Sinauer; Text material © 2025 by Steven M. Carr