- Genetic variance is the component of phenotypic variance that results from genetic differences among individuals in a population. It describes how much of the observed variation in a trait is associated with differences in genotypes. Genetic variance is a fundamental concept in quantitative genetics, population genetics, evolutionary genetics, and plant and animal breeding because it helps explain inheritance, response to selection, and the genetic basis of complex traits.
- Genetic variance is commonly represented by Vᴳ or VG. When individuals in a population carry different alleles or combinations of alleles that affect a trait, their genetic values can differ. These genetic differences contribute to variation in observable phenotypes, although environmental factors also influence the traits that are ultimately measured.
- A simplified model separates phenotypic variance into genetic and environmental components: Vₚ = Vᴳ + Vᴱ
- where Vₚ is phenotypic variance, Vᴳ is genetic variance, and Vᴱ is environmental variance. This is a useful conceptual model, although more complete statistical models may include covariance and interaction terms.
- Genetic variance can be further divided into several components. A commonly used quantitative-genetics model is: Vᴳ = Vᴬ + Vᴰ + Vᴵ
- where Vᴬ is additive genetic variance, Vᴰ is dominance variance, and Vᴵ is epistatic variance. These components arise from different aspects of genetic effects and are important for understanding how genetic differences are inherited and expressed.
- Additive genetic variance results from the cumulative effects of individual alleles and is especially important for predicting resemblance between parents and offspring. Because additive effects are transmitted relatively predictably across generations, additive variance is the primary genetic component underlying the expected response to selection.
- Dominance variance results from interactions between alleles at the same genetic locus. The phenotype associated with a heterozygous genotype may differ from what would be expected from simply adding the effects of the two alleles. Dominance effects contribute to genetic variance but are not transmitted to offspring in the same predictable way as additive effects.
- Epistatic variance arises from interactions between genetic effects at different loci. One allele or genotype can influence the effect of another locus, creating genetic interactions that contribute to complex trait variation. Epistasis is particularly important when studying traits with complicated genetic architecture.
- The relative contribution of these components depends on the population being studied. Genetic variance is influenced by allele frequencies, genetic diversity, linkage disequilibrium, population structure, and the genetic architecture of the trait. Therefore, genetic variance is not a universal constant for a trait and can differ among populations or environments.
- Genetic variance is closely connected to heritability. Broad-sense heritability is defined as the proportion of phenotypic variance attributable to total genetic variance: H² = Vᴳ / Vₚ
- In contrast, narrow-sense heritability measures the proportion of phenotypic variance attributable specifically to additive genetic variance: h² = Vᴬ / Vₚ
- The distinction between broad- and narrow-sense heritability is important because total genetic variance and additive genetic variance have different implications for inheritance and selection.
- A population may have substantial genetic variance but relatively little additive genetic variance. In such a case, individuals can differ genetically, but a smaller proportion of those differences may be transmitted predictably from parents to offspring. Conversely, a population with substantial additive genetic variance may have greater potential for response to selection.
- Genetic variance is therefore central to natural selection. Natural selection acts on differences in phenotype, but evolutionary change depends on the extent to which those differences are genetically determined and transmitted across generations. When phenotypic differences associated with fitness contain sufficient additive genetic variance, allele frequencies and trait distributions can change over time.
- The breeder’s equation, R = h²S, illustrates the relationship between heritability and response to selection. Here, R represents the expected response to selection and S represents the selection differential. Because narrow-sense heritability depends on additive genetic variance, additive variance is particularly important for predicting genetic improvement or evolutionary response.
- Genetic variance is also fundamental to the concept of breeding value. The breeding value of an individual represents the expected contribution of its genes to the performance of its offspring under a defined population and model. Because breeding values are primarily based on additive genetic effects, additive genetic variance determines much of the predictable genetic variation used in breeding programs.
- In quantitative traits, genetic variance usually results from the combined effects of many genes and genomic regions. Traits such as height, body weight, crop yield, growth rate, milk production, disease resistance, and many physiological characteristics can have complex genetic architectures involving numerous loci with different effect sizes.
- Polygenic inheritance is therefore closely associated with genetic variance. When many genes contribute to a trait, differences in allele combinations among individuals can produce a broad distribution of genetic values. Environmental effects then contribute additional phenotypic variation, producing the observed trait distribution in the population.
- Genetic architecture describes the number, locations, effect sizes, interactions, and other characteristics of genetic factors contributing to a trait. Additive effects, dominance, epistasis, pleiotropy, linkage, and linkage disequilibrium can all influence the amount and structure of genetic variance.
- Genetic variance can also contribute to genetic covariance between traits. When some genetic factors influence multiple traits, individuals may show genetically correlated differences in those traits. This can produce positive or negative genetic correlations, which are important in multivariate quantitative genetics and breeding.
- For example, selection for increased production of one trait may unintentionally affect another genetically correlated trait. Understanding genetic covariance and the G-matrix helps researchers predict how populations may respond to simultaneous selection on multiple characteristics.
- Genetic variance is affected by allele frequencies. A genetic variant can contribute strongly to variance when different alleles occur at meaningful frequencies in a population. If an allele becomes extremely rare or fixed, its contribution to population-level genetic variance may decrease or disappear, even though the allele can remain biologically important.
- Mutation introduces new genetic variants and can therefore provide the ultimate source of new genetic variation. Genetic recombination, crossing over, independent assortment, and gene flow can rearrange or redistribute existing variation. Genetic drift, natural selection, and other evolutionary forces can subsequently alter allele frequencies and the amount of genetic variance within populations.
- Population size also influences genetic variance. Small populations are more strongly affected by genetic drift, which can cause alleles to be lost or fixed. Population bottlenecks and founder effects can therefore reduce genetic diversity and potentially reduce the genetic variance available for future adaptation.
- Inbreeding can also alter the genetic composition of populations. Increased homozygosity can expose deleterious recessive variants and contribute to inbreeding depression. In breeding populations, maintaining appropriate levels of genetic diversity can be important for preserving genetic variance and long-term improvement.
- Genetic variance is an important consideration in plant and animal breeding. Breeders need genetic differences among individuals to make selection effective. If all individuals have nearly identical genetic values for a trait, selection has little opportunity to produce genetic improvement, even when substantial environmental variation exists.
- Modern breeding programs use genomic selection and genomic prediction to estimate genetic potential from genome-wide markers. Genomic estimated breeding values (GEBVs) allow breeders to identify individuals with favorable genetic profiles before complete phenotypic information is available. These methods can increase selection efficiency for complex traits.
- Genetic variance can also be investigated using QTL mapping and genome-wide association studies (GWAS). QTL analyses identify genomic regions associated with variation in quantitative traits, while GWAS tests associations between genetic variants and phenotypic differences across many individuals. Together, these approaches can help identify genetic factors contributing to complex traits.
- However, genetic variance does not necessarily mean that every associated genetic factor has a simple or independent effect. Epistasis, pleiotropy, linkage disequilibrium, population structure, and environmental effects can influence the relationship between genetic variants and phenotypes. Consequently, estimates of genetic variance depend on the population, environment, statistical model, and trait definition.
- Genotype–environment interaction (G×E) can further complicate the interpretation of genetic variance. Different genotypes may respond differently to environmental conditions, meaning that genetic differences observed in one environment may not have the same effects in another. This is particularly important in agricultural breeding and studies of adaptation.
- Genetic variance is also relevant to evolutionary genetics because it determines part of a population’s potential to respond to changing selection pressures. Populations with sufficient additive genetic variance may have greater potential for evolutionary change when environmental conditions or fitness landscapes change.
- In conservation biology, maintaining genetic variance can help preserve the adaptive potential of threatened populations. Genetic diversity provides the raw material on which future selection can act, making the maintenance of healthy population sizes and genetic connectivity important for long-term evolutionary resilience.
- In human genetics, genetic variance contributes to differences among individuals in many complex traits and disease-related characteristics. Researchers use family studies, twin studies, pedigrees, population data, and genomic information to estimate genetic contributions to phenotypic differences. These estimates describe variation within populations and should not be interpreted as fixed genetic percentages for individual people.
- An important distinction is that genetic variance is a population-level statistical concept. It does not mean that a specific individual has a certain percentage of genetic variance for a trait. Instead, it describes how much individuals differ genetically within a defined population and contributes to understanding why their phenotypes differ.
- Genetic variance also depends on the scale and context of measurement. A trait can have different estimates of genetic variance in different populations, environments, age groups, or developmental stages. Changes in allele frequencies, environmental conditions, genetic diversity, and population structure can all alter the observed variance components.
- Overall, genetic variance provides the central connection between genetic variation and differences in phenotypes. By separating genetic variance into additive, dominance, and epistatic components, quantitative genetics provides a framework for understanding inheritance, heritability, breeding, natural selection, adaptation, and the genetic architecture of complex traits.
