 Underdominant allele-frequency dynamics

With homozygote fitness advantages $a,b>0$ over the heterozygote, the allele frequency $u$ of the first allele has an unstable threshold $b/(a+b)$. Below it that allele decreases; above it it increases. In a discrete viability-selection generation the same numerator gives $u^{\prime }-u$ after division by mean genotype fitness. Equal homozygote advantages give $\dot{u}=su(1-u)(2u-1)$, with stable endpoints and unstable midpoint.