Partial dominance hypothesis¶
A population-genetic hypothesis explaining inbreeding depression as increased homozygous expression of deleterious alleles whose harmful effects are recessive or partially recessive in heterozygotes.
Core Idea¶
The partial dominance hypothesis explains inbreeding depression as the increased homozygous expression of deleterious alleles whose harmful effects are recessive or partially recessive in heterozygotes. Inbreeding raises identity by descent and homozygosity, exposing more of the population’s masked genetic load and reducing fitness or a fitness-related trait.
Under continued inbreeding and selection, deleterious alleles may be purged from some lines, fixed in others, or contribute to line extinction. This possible longer-run response distinguishes the mechanism from a static statement that homozygosity is merely correlated with low fitness.
Structural Signature¶
- Inbreeding regime raises identity by descent and homozygosity.
- Deleterious allele load supplies segregating negative fitness effects.
- Partial recessivity masks some effect in heterozygotes and exposes it in homozygotes.
- Fitness response supplies the depression to be explained.
- Generation and line trajectory reveal purging, fixation, or loss over time.
What It Is Not¶
The hypothesis is not the overdominance hypothesis, which attributes depression to the loss of superior heterozygous genotypes. It is not every adverse outcome in a small population, because drift, demographic deterioration, environment, and mating structure can also reduce performance. Outcrossing recovery alone does not uniquely prove partial dominance.
Scope of Application¶
The hypothesis is evaluated in population genetics, quantitative genetics, conservation, breeding, and experimental inbreeding studies. It applies to fitness components and traits influenced by segregating deleterious variants. Tests require designs that distinguish masked load from heterozygote advantage and from non-genetic changes.
Clarity¶
The abstraction separates trigger, genetic substrate, dominance relation, and phenotypic consequence. “Inbreeding increases homozygosity” is only the trigger; the hypothesis adds that harmful alleles were partly masked and become expressed. That causal statement yields predictions about allele frequencies and line trajectories.
Manages Complexity¶
Many loci of small effect, dominance coefficients, linkage, selection, and stochastic loss contribute to inbreeding depression. The hypothesis compresses them into a directional mechanism: masked deleterious load becomes homozygous. The compression is useful but does not justify assuming one dominance coefficient or ignoring line extinction and ascertainment.
Abstract Reasoning¶
Measure trait or fitness change under known inbreeding, compare replicate lines and generations, and model the expected increase in homozygosity. Seek evidence of deleterious variants, dominance, purging, fixation, and differential line survival. Compare predictions from partial dominance with overdominance and demographic alternatives. Treat restored fitness after crossing as compatible with several mechanisms unless additional genetic evidence discriminates them.
Knowledge Transfer¶
The hypothesis transfers across taxa and traits only when inbreeding, load, dominance, and fitness are measured comparably. The causal logic informs conservation and breeding, but parameter estimates do not move unchanged across populations. Outside genetics, “partial dominance” may name a phenotype relationship and should not automatically invoke this population-level explanation.
Examples¶
Canonical¶
Replicate inbred lines decline as harmful recessives become homozygous; surviving lines later show evidence of purging while others fix load or disappear.
Mapped back: regime → repeated inbreeding; load → segregating harmful alleles; dominance → partial recessivity; response → fitness decline; trajectory → purging, fixation, and line loss.
Applied / In Practice¶
Crosses among inbred lines regain fitness, but marker, pedigree, and multigeneration data are used before attributing that rebound specifically to partial dominance.
Structural Tensions¶
Short-term depression versus long-term purging. Exposure lowers current fitness while selection can remove harmful alleles from survivors. Diagnostic: Does apparent recovery reflect within-line purging, extinction of weak lines, or environmental change?
Partial dominance versus overdominance. Both can produce depression and heterosis while implying different locus-level mechanisms. Diagnostic: Which observations distinguish masked deleterious load from heterozygote advantage?
Structural–Framed Character¶
Partial Dominance Hypothesis is structural as a genotype-frequency and expression mechanism. It is framed by population-genetic definitions of fitness, dominance, inbreeding, and evidence from particular pedigrees and environments.
Structural Core vs. Domain Accent¶
The core is inbreeding → homozygosity → expression of masked harm → fitness decline, with selection modifying later trajectories. The domain accent supplies alleles, dominance coefficients, pedigrees, and fitness measures.
Instantiates / Related Primes¶
- Approved unparented root. No live causal-hypothesis parent captures this mechanism.
- Masking explains heterozygous concealment.
- Selection drives purging and line survival.
- Causation is asserted and must be distinguished from association.
Neighborhood in Abstraction Space¶
Partial dominance hypothesis sits in a sparse region of the domain-specific corpus (96th 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
- Wallace Effect — 0.79
- Hardy-Weinberg Principle — 0.78
- Vicar of Bray (scientific hypothesis) — 0.77
- Haldane's Sieve — 0.77
- Fixation (population genetics) — 0.77
Computed from structural-signature embeddings · 2026-10-08
Not to Be Confused With¶
- Overdominance hypothesis: loss of heterozygote advantage.
- Dominance phenotype: an allele-level expression pattern without the population explanation.
- Genetic drift: stochastic frequency change that may accompany but does not define the mechanism.
- Heterosis: crossing response compatible with more than one cause.
References¶
- Derek A. Roff, “Inbreeding Depression: Tests of the Overdominance and Partial Dominance Hypotheses,” Evolution 56(4), 2002, DOI: 10.1111/j.0014-3820.2002.tb01387.x.
- Frozen Wikipedia discovery revision: https://en.wikipedia.org/wiki/Partial_dominance_hypothesis