- Dominance Variance is a component of genetic variance that arises from interactions between different alleles at the same genetic locus. It is an important concept in quantitative genetics because it helps explain differences between individuals for complex traits and distinguishes genetic effects that are predictable through additive inheritance from effects caused by allele combinations within loci.
- When a diploid organism carries two alleles at a locus, the phenotype associated with the genotype may not equal the simple sum of the effects of the two alleles. The interaction between alleles in a heterozygous genotype is called dominance. Dominance variance describes the contribution of these within-locus interactions to variation among individuals in a population.
- Dominance variance is commonly denoted by VDV_D. In a simplified decomposition of phenotypic variance, genetic variance can be represented as additive variance, dominance variance, and epistatic variance, together with environmental variance. A simplified expression is VP=VA+VD+VI+VEV_P = V_A + V_D + V_I + V_E, although real quantitative-genetic models may include additional covariance and interaction components.
- The distinction between additive genetic variance and dominance variance is fundamental. Additive genetic effects represent the average contributions of alleles and are transmitted from parents to offspring in a statistically predictable way. Dominance effects arise from combinations of alleles within a locus and are therefore generally less predictable across generations.
- For a locus with two alleles, often represented as AA and aa, the three possible genotypes are AAAA, AaAa, and aaaa. Under complete dominance, the heterozygous genotype AaAa may have a phenotype similar to AAAA. Under incomplete dominance, the heterozygote may have an intermediate phenotype, while under overdominance the heterozygote may have a phenotype exceeding both homozygotes.
- These different patterns of allele interaction contribute to the genetic architecture of traits. Dominance deviation describes the departure of a heterozygous genotype from the value expected under an additive model. When dominance deviations vary among individuals because individuals carry different genotype combinations, they contribute to dominance variance.
- Dominance variance depends on allele frequencies. The amount of dominance variance for a trait can change when allele frequencies change, even if the biological effects of the alleles themselves remain unchanged. Consequently, dominance variance is a population-specific statistical quantity rather than a fixed property of a particular gene.
- For a simple two-allele locus, dominance variance is influenced by the frequencies of the two alleles and the magnitude of the dominance deviation. In simplified quantitative-genetic models, dominance variance is often expressed using allele frequencies and dominance effects. With many loci contributing to a trait, the total dominance variance represents the combined contribution of dominance effects across those loci.
- Dominance variance is particularly important when populations contain substantial numbers of heterozygous individuals. If a population becomes highly homozygous, dominance effects may contribute less to phenotypic variation because fewer individuals carry different alleles at the same loci.
- This relationship makes dominance variance relevant to inbreeding. Inbreeding increases homozygosity and decreases heterozygosity. As heterozygosity changes, the contribution and expression of dominance effects can also change. Inbreeding can therefore alter the phenotypic expression of traits influenced by dominance.
- The phenomenon known as inbreeding depression is closely related to dominance and genetic load. Harmful recessive alleles can remain hidden in heterozygous individuals but become more frequently expressed as homozygotes when inbreeding increases. This can reduce survival, fertility, growth, or other components of fitness.
- Dominance is also important in understanding heterosis, or hybrid vigor. In some crosses, offspring perform better than their parents for traits such as growth, fertility, yield, or disease resistance. Dominance and other genetic mechanisms can contribute to heterosis, although its genetic basis may also involve overdominance, epistasis, and other forms of genetic interaction.
- Dominance variance should not be confused with epistatic variance. Dominance describes interactions between alleles at the same locus, whereas epistasis describes interactions between alleles or genetic effects at different loci. Both are forms of non-additive genetic variation, but they represent different biological relationships.
- For example, if the effect of allele AA depends on whether it is paired with allele aa at the same locus, the interaction contributes to dominance. If the effect of a variant at one locus depends on the genotype at another locus, the interaction represents epistasis.
- Dominance variance contributes to phenotypic variation, but it does not contribute to narrow-sense heritability in the same way as additive genetic variance. Narrow-sense heritability is defined as the proportion of phenotypic variance attributable to additive genetic variance:
- h2=VA/VPh^2 = V_A / V_P
- Dominance variance is included in broad-sense heritability, which considers total genetic variance:
- H2=VG/VPH^2 = V_G / V_P
- where VGV_G may include additive, dominance, and epistatic components.
- This distinction is important when predicting the response to selection. The breeder’s equation, R=h2SR = h^2S, uses narrow-sense heritability because the predictable transmission of additive genetic effects is primarily responsible for the resemblance between parents and offspring and the long-term response to selection.
- Dominance effects can nevertheless influence short-term selection responses. Selected individuals may carry favorable allele combinations that include dominance effects, and these combinations can contribute to their observed phenotypes. However, the specific allele combinations inherited by their offspring are partly reshuffled through meiosis and Mendelian segregation.
- Because dominance effects depend on genotype combinations, offspring do not necessarily inherit their parents’ dominance deviations. Two individuals with similar phenotypes can have different combinations of alleles and therefore different breeding values and dominance effects.
- This is why breeding value and genotypic value should be distinguished. Breeding value describes the additive genetic contribution expected to be transmitted to offspring, whereas genotypic value represents the total genetic effect associated with an individual’s genotype, including additive, dominance, and potentially epistatic effects.
- Dominance variance is therefore particularly important in breeding programs where hybrid performance is important. Commercial hybrid breeding in crops, poultry, livestock, and other organisms may exploit favorable combinations of alleles that produce strong dominance effects.
- In plant breeding, dominance can influence traits such as yield, biomass, fertility, disease resistance, and stress tolerance. Breeders may use specific parental combinations to produce hybrids that outperform their parental lines. The usefulness of such crosses depends partly on the genetic architecture of the target trait.
- In animal breeding, dominance effects may influence growth, fertility, survival, production traits, and other economically important characteristics. Although additive genetic variance is usually central to long-term breeding programs, dominance effects may become particularly important when evaluating crosses or designing mating strategies.
- Modern genomic selection can incorporate dominance effects into statistical prediction models. Genomic data from large numbers of markers can be used to estimate additive and non-additive genetic effects. Models that include dominance can sometimes improve predictions when dominance contributes substantially to the trait being evaluated.
- A genomic relationship matrix can be constructed to represent additive genetic relationships among individuals, while specialized genomic models can also estimate dominance relationships. These approaches allow researchers to investigate how genome-wide genotype combinations contribute to phenotypic differences.
- Dominance variance can also be studied using quantitative trait loci (QTL) and genome-wide association studies (GWAS). Some QTL may show evidence of additive effects, while others may display dominance or other non-additive effects. Identifying these patterns helps researchers understand the genetic architecture of complex traits.
- The contribution of dominance variance can vary across populations and environments. Genotype–environment interaction (G×E) may cause dominance effects to differ depending on environmental conditions. A heterozygous genotype may have an advantage in one environment but not another, depending on how alleles interact with environmental factors.
- Dominance is also relevant to evolutionary genetics because it influences how natural selection acts on genetic variants. A beneficial or harmful allele can behave differently depending on whether it is dominant, recessive, partially dominant, or overdominant. The rate at which an allele changes in frequency can therefore depend strongly on its dominance relationship.
- A harmful recessive allele, for example, may persist in a population because its effects are masked in heterozygotes. Natural selection is less effective at removing an allele when its harmful effect is largely hidden in heterozygous individuals. In contrast, a harmful dominant allele can be exposed to selection even when present in a single copy.
- Dominance can therefore influence the maintenance of genetic variation within populations. The interaction between selection, allele frequencies, mutation, migration, genetic drift, and dominance determines how genetic variants are maintained or removed over evolutionary time.
- Dominance variance can also affect genetic correlations between traits. If the same genetic loci contribute dominance effects to multiple traits, correlations between traits may arise through shared genetic mechanisms. Multivariate quantitative-genetic models can incorporate these covariance components.
- The magnitude of dominance variance should always be interpreted within a defined population and environmental context. Changes in allele frequencies, genetic background, mating structure, or environmental conditions can alter the estimated contribution of dominance to phenotypic variation.
- Dominance variance is ultimately one component of the broader genetic architecture of quantitative traits. Along with additive genetic variance, epistatic variance, and environmental variance, it helps explain why individuals differ in phenotype and why genetic effects can sometimes be difficult to predict from individual alleles alone.
- Understanding dominance variance is essential for interpreting heritability, breeding value, genetic architecture, heterosis, inbreeding depression, hybrid breeding, and quantitative genetics. It provides an important framework for understanding how interactions between alleles at the same locus contribute to biological variation and influence inheritance, breeding, and evolution.
