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Heterozygote disadvantage in reproductive success is called underdominance. In a single locus system with two or more alleles, this leads to bistable evolutionary dynamics. Reciprocal balanced chromosomal translocations occur naturally and can lead to underdominance. The coexistence of two alleles that are in underdominance with each other is unstable under Darwinian selection. However, with other evolutionary mechanisms, an underdominant polymorphism can be stabilized, for example, in a spatially extended population by a selection–migration equilibrium.