Underdominance¶
A population-genetic fitness ordering in which a heterozygote is less fit than either corresponding homozygote in a declared context.
Core Idea¶
Underdominance means that a heterozygote has lower fitness than both corresponding homozygotes in the same declared setting. For genotypes \(AA\), \(AB\) and \(BB\), its defining comparison is \(w_{AB}<w_{AA}\) and \(w_{AB}<w_{BB}\). The environment and fitness measure must be common to all three terms; one may be discussing total fitness or a stated component such as pollen fertility.[ref-091adf2b7cab][ref-45c98cb6ad10]
In a simple isolated-population model, this disadvantage produces an unstable interior allele-frequency equilibrium, so an allele's starting frequency matters. That familiar result is not the definition. Migration and population-network topology can alter stability and allow differentiated selection–migration equilibria while the local heterozygote disadvantage remains.[^ref-091adf2b7cab]
Scope of Application¶
In natural cytogenetics, Stathos and Fishman found underdominant male pollen sterility associated with particular heterozygous reciprocal translocations in Mimulus hybrid comparisons. Their result supports a fertility-component claim, not a measurement of all lifetime fitness or observed allele fixation.[^ref-45c98cb6ad10]
In population modeling, Láruson and Reed place the same three-genotype ordering in connected demes. Their comparison shows why the local fitness relation must be separated from model-dependent predictions about frequencies across a network. Equal homozygote fitness is their simplifying special case, not a requirement of underdominance.[^ref-091adf2b7cab]
Clarity¶
The two inequalities answer which genotype lies below both alternatives? A heterozygote below only one homozygote does not meet the strict pattern. A low pollen-fertility measurement should be described as such rather than silently upgraded to total fitness; a predicted threshold should be tied to its mating and migration assumptions.[ref-091adf2b7cab][ref-45c98cb6ad10]
Manages Complexity¶
One double inequality compresses a three-genotype fitness configuration without requiring equal homozygote values. Keeping that configuration separate from population structure lets the same local relation be examined in an isolated population and a connected-deme model without copying one model's outcome into the other.[^ref-091adf2b7cab]
Abstract Reasoning¶
First identify corresponding \(AA\), \(AB\) and \(BB\) cases and a common environment and fitness metric. Compare \(AB\) against each endpoint. If both strict inequalities hold, state the supported level—component or total fitness. Only then select an appropriate model to ask whether frequencies will change, remain differentiated or fluctuate under further forces.[ref-091adf2b7cab][ref-45c98cb6ad10]
Knowledge Transfer¶
The genotype-fitness test transfers within population genetics from allelic models to chromosomal-arrangement studies when the three genotype roles and fitness comparison stay literal. A natural pollen-fertility result can inform a model but does not by itself validate the model's fixation prediction. Outside genetics, “the mixture is worse than either pure option” is an analogy or a possible future-prime skeleton, not literal underdominance.
[^ref-091adf2b7cab]: Aki J. Láruson and Floyd A. Reed, “Stability of Underdominant Genetic Polymorphisms in Population Networks”, original research preprint, v2 (2015), abstract, §1 and §2.1; full PDF inspected 2026-10-01. [^ref-45c98cb6ad10]: Angela Stathos and Lila Fishman, “Chromosomal rearrangements directly cause underdominant F1 pollen sterility in Mimulus lewisii–Mimulus cardinalis hybrids”, Evolution 68 (2014), 3109–3119, abstract and introduction; full PDF inspected 2026-10-01.
Neighborhood in Abstraction Space¶
Underdominance sits in a sparse region of the domain-specific corpus (63rd percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.
Family — Selection, Speciation & Experimental Evolution (22 abstractions)
Nearest neighbors
- Genetic Load — 0.86
- Wallace Effect — 0.86
- Co-adaptation — 0.85
- Kin selection — 0.84
- Trivers–Willard Hypothesis — 0.84
Computed from structural-signature embeddings · 2026-10-08