- Genetic gain is the improvement in the genetic value of a population that occurs as a result of selection across generations. It is a central concept in quantitative genetics, animal breeding, plant breeding, and genetic improvement programs. Genetic gain occurs when individuals with desirable genetic characteristics are preferentially chosen as parents, causing the frequency of favorable genetic variants or combinations of variants to increase in subsequent generations.
- Genetic gain should be distinguished from a simple change in the observed phenotype. A population may become taller, heavier, faster-growing, or more productive because of improved nutrition, management, climate, or other environmental conditions without experiencing a corresponding genetic change. True genetic gain refers to improvement in the population’s genetic merit, although it is often observed indirectly through changes in phenotypic performance under comparable conditions.
- One of the most important relationships in quantitative genetics is the Breeder’s Equation, which expresses genetic response to selection as R=h2SR = h^2S. Here, RR is the selection response, h2h^2 is narrow-sense heritability, and SS is the selection differential. Because narrow-sense heritability represents the proportion of phenotypic variance attributable to additive genetic variance, the equation shows why genetic gain depends on both the strength of selection and the amount of heritable genetic variation available.
- Genetic gain can also be expressed using selection intensity, selection accuracy, and additive genetic variation. A simplified expression for genetic gain per generation is ΔG≈irσA\Delta G \approx i r \sigma_A, where ii is selection intensity, rr is selection accuracy, and σA\sigma_A is the additive genetic standard deviation. This formulation is particularly useful for modern breeding programs because it shows that genetic improvement depends not only on how strongly individuals are selected but also on how accurately their genetic merit is predicted.
- The distinction between genetic gain and selection response is useful. Selection response generally describes the change in the population mean from one generation to the next as a result of selection. Genetic gain is often used more broadly for the improvement in genetic merit achieved through a breeding program and may be expressed per generation or per unit of time. In practical breeding programs, genetic gain per year is especially important because breeding programs differ greatly in how quickly they produce new generations.
- Generation interval therefore plays an important role in genetic gain. A breeding program that produces the same genetic improvement per generation but completes generations more rapidly can achieve greater improvement per year. A simplified relationship is:
- ΔGyear≈irσAL\Delta G_{\text{year}} \approx \frac{i r \sigma_A}{L}
- where LL is the generation interval. This equation illustrates why breeders seek an appropriate balance between selection intensity, selection accuracy, additive genetic variation, and generation interval.
- For example, suppose a breeding program produces a genetic improvement of 2 units per generation and has a generation interval of 4 years. Its approximate genetic gain is 0.5 units per year. If another strategy produces the same 2 units of improvement per generation but reduces the generation interval to 2 years, the gain becomes approximately 1 unit per year. The underlying response per generation has not changed, but the rate of genetic improvement has doubled.
- The amount of additive genetic variation available in a population places an important limit on genetic gain. Additive genetic variation represents genetic differences that can be transmitted predictably from parents to offspring. If additive genetic variance is large, there may be substantial opportunity for selection to produce genetic improvement. If additive genetic variance becomes depleted, continued selection may produce progressively smaller gains unless new genetic variation enters the population through mutation, migration, recombination, or the introduction of genetically diverse breeding material.
- Heritability also affects expected genetic gain. Traits with higher narrow-sense heritability generally provide greater opportunity for response to individual phenotypic selection because a larger proportion of phenotypic differences is associated with additive genetic differences. However, heritability is population- and environment-specific and should not be interpreted as a fixed property of a trait. A low-heritability trait can still achieve substantial genetic gain when breeders use large amounts of family, progeny, repeated-record, or genomic information.
- Selection intensity determines how strongly individuals are selected. Selecting a smaller proportion of individuals generally increases selection intensity and can increase expected genetic gain per generation. However, very intense selection can have undesirable consequences, including increased inbreeding, reduced effective population size, loss of genetic diversity, and reduced long-term breeding potential. Consequently, maximizing short-term genetic gain is not always equivalent to maximizing sustainable long-term improvement.
- Selection accuracy is another major determinant of genetic gain. Accuracy describes how closely the information used for selection predicts true breeding value. If accuracy is low, individuals may be selected because they appear genetically superior when their performance is actually influenced strongly by environmental effects. Higher accuracy increases the probability that selected individuals truly have favorable genetic merit.
- The development of genomic selection has provided new opportunities to increase genetic gain. Genomic selection uses genome-wide marker information to estimate genetic merit, often producing genomic estimated breeding values (GEBVs). Because young individuals can be evaluated before they have accumulated extensive phenotypic or progeny information, genomic selection can increase selection accuracy for some traits while substantially reducing generation interval. The combination of higher accuracy and shorter generation interval can increase annual genetic gain.
- Genomic selection is particularly valuable for traits that are difficult or expensive to measure. Examples include disease resistance, feed efficiency, fertility, longevity, carcass characteristics, and traits that are expressed late in life. Instead of waiting until an individual has expressed the trait or produced offspring, breeders may use genomic information to estimate genetic merit earlier in life.
- Breeding value is central to genetic gain because selection aims to identify individuals whose genetic contributions will improve the next generation. The breeding value of an individual represents the expected additive genetic contribution of that individual to its offspring. Breeders estimate breeding values using phenotypic records, pedigree relationships, relatives, progeny, genomic information, or combinations of these sources.
- Modern breeding programs often use BLUP and related mixed-model methods to estimate breeding values. These approaches can simultaneously account for fixed environmental effects, relationships among individuals, and random genetic effects. By separating genetic and environmental sources of variation more effectively, they can improve the reliability of selection decisions and therefore contribute to genetic gain.
- Genetic gain can be measured for a single trait or across multiple traits. Most real breeding programs have several objectives, such as improving growth while maintaining fertility, health, product quality, and survival. Genetic correlation and genetic covariance become important because selection for one trait can cause correlated changes in another trait. A favorable genetic correlation can help achieve simultaneous improvement, whereas an unfavorable correlation may create a trade-off.
- A selection index provides a systematic approach for combining information from several traits into a single selection criterion. The index can incorporate economic values, genetic covariances, phenotypic information, pedigree information, and genomic information. By optimizing selection across multiple traits, breeders can pursue balanced genetic gain rather than maximizing improvement in a single trait at the expense of other important characteristics.
- The concept of genetic gain is also closely connected to genetic architecture. Traits controlled by many genes of small effect may respond differently to selection than traits influenced by a few genes of larger effect. The distribution of genetic effects, allele frequencies, linkage disequilibrium, pleiotropy, dominance, and epistasis can all influence the amount and persistence of genetic improvement.
- For complex traits, genetic gain may not remain constant over time. As favorable alleles become more common, the available genetic variation can decline, and the response to selection may slow. This phenomenon is sometimes described as a diminishing response to selection. Maintaining genetic diversity can therefore be important for preserving long-term opportunities for genetic improvement.
- The source of genetic variation also matters. Mutation introduces new genetic variants, while recombination creates new combinations of existing variants. Gene flow can introduce genetic variation from other populations. Breeding programs may deliberately introduce genetically diverse material to increase variation, although new genetic material must be evaluated carefully because it may also introduce undesirable traits.
- Environmental effects must also be controlled when evaluating genetic gain. Changes in nutrition, management, housing, disease exposure, technology, or climate can produce phenotypic improvements that might otherwise be mistaken for genetic progress. Long-term breeding programs therefore use standardized testing, contemporary groups, statistical models, and genetic evaluation systems to separate genetic trends from environmental trends.
- Genotype–environment interaction (G×E) can complicate the measurement of genetic gain. Genetic performance may differ across locations, climates, management systems, or production environments. A genotype that performs well in one environment may not perform equally well elsewhere. If the target population includes multiple environments, breeding programs may need to evaluate genetic performance across those environments to ensure that measured genetic gain is relevant to the intended production conditions.
- Genetic gain is also influenced by maternal effects, common environmental effects, and permanent environmental effects. If these sources of variation are not properly accounted for, environmental differences may be incorrectly attributed to genetic differences. Appropriate statistical models help distinguish direct additive genetic effects from non-genetic effects and improve estimates of breeding value and genetic trend.
- Genetic gain can be evaluated retrospectively by comparing the genetic merit of populations or cohorts across time. This is sometimes called genetic trend or genetic progress. A positive genetic trend indicates that the average genetic merit of the population is increasing for the trait of interest. Genetic trends are particularly useful for evaluating whether a breeding program is achieving its intended objectives.
- In conservation breeding and small populations, genetic gain must be balanced against the preservation of genetic diversity. Strong selection can produce rapid short-term improvement but may increase inbreeding and reduce the effective population size. Maintaining sufficient genetic diversity allows populations to retain evolutionary potential and reduces the risk associated with excessive accumulation of harmful recessive variants.
- Genetic gain is not limited to artificial selection. Natural selection can also produce genetic change when heritable differences in traits influence survival or reproductive success. In evolutionary genetics, changes in mean genetic value across generations can therefore be viewed as evolutionary responses to selection. The same quantitative-genetic principles involving genetic variation, heritability, selection intensity, and environmental effects help explain evolutionary change.
- The most efficient breeding strategy depends on the biological and economic characteristics of the breeding program. Increasing selection intensity can raise short-term gain, while increasing selection accuracy can improve the reliability of selection. Reducing generation interval can accelerate gain per year, and maintaining genetic diversity can support long-term progress. Modern breeding therefore focuses on optimizing these factors together rather than maximizing any single component.
- Genetic gain is ultimately a measure of how effectively a breeding or selection program converts heritable genetic variation into sustained improvement. Its magnitude depends on the amount of additive genetic variation, selection intensity, selection accuracy, generation interval, genetic correlations among traits, and the quality of genetic evaluation. Understanding these relationships provides the foundation for modern genetic improvement, genomic selection, animal and plant breeding, and the study of evolutionary change.
