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A heterozygote has greater genotype fitness than either corresponding homozygote. In the deterministic random-mating two-allele viability-selection model this gives an attracting interior allele frequency, unlike the repelling interior frequency produced by underdominance.

Let $u$ be the allele frequency of $A$, with random mating and sufficiently large population size to neglect genetic drift. The Hardy-Weinberg proportions before selection are $u^2,2u(1-u),(1-u{)}^2$. Write the three genotype fitnesses as $w_{AA},w_{Aa},w_{aa}$, with $w_{Aa}$ smaller than either homozygote value. After one viability-selection generation,ConsequentlyPut $a=w_{AA}-w_{Aa}>0$, $b=w_{aa}-w_{Aa}>0$. The underdominant allele-frequency dynamics has stable fixation states zero and one, separated by the unstable thresholdBelow it $A$ decreases to allele fixation of $a$; above it $A$ increases to fixation. Exactly at the threshold the deterministic frequency remains balanced, but any perturbation selects a side. This is underdominance, or heterozygote disadvantage, rather than balancing selection from heterozygote advantage.

In a continuous-time weak-selection or selection-rate convention, the corresponding equation is $\dot{u}=u(1-u)[(a+b)u-b]$. Equal homozygote fitness gives $\dot{u}=su(1-u)(2u-1)$ for a positive selection strength $s$. The next part uses this explicitly stated continuous-time convention; $s$ and migration then have the same time units.

Underdominance means that the heterozygote has lower genotype fitness than both corresponding homozygotes. In a deterministic randomly mating biallelic population, it produces an unstable interior allele-frequency threshold separating the two fixation outcomes, unlike heterozygote advantage.