- True breeding value is a fundamental concept in quantitative genetics and animal breeding that describes the actual additive genetic merit of an animal for a particular trait. It represents the animal’s underlying genetic potential to transmit favourable or unfavourable alleles to its offspring and is the genetic quantity that breeders ultimately want to know when making selection decisions. Because the true breeding value cannot normally be observed directly, it must be estimated from available information such as the animal’s own performance, relatives, progeny, pedigree, genomic information, and records for genetically correlated traits.
- The concept of true breeding value is closely related to the distinction between phenotypic value, genotypic value, and breeding value. An animal’s phenotype is the measurable expression of a trait and is influenced by both genetic and environmental factors. This relationship can be represented as P = G + E, where P is phenotypic value, G is genetic value, and E represents environmental effects. Genetic value can contain additive effects, dominance effects, and epistatic effects, whereas the breeding value specifically represents the additive genetic component that can be transmitted predictably from parents to offspring.
- The true breeding value therefore describes the animal’s underlying additive genetic contribution rather than its temporary environmental advantage. For example, two dairy cows may produce different quantities of milk because they experience different feeding, health, housing, or management conditions. The cow producing more milk does not necessarily have the higher true breeding value. If the lower-producing cow has a more favourable additive genetic constitution but experienced poorer environmental conditions, its true breeding value may actually be higher.
- The word true is important because the true breeding value is a theoretical genetic quantity rather than a directly observed measurement. In practice, breeders do not know the exact true breeding value of an animal. Instead, they use statistical and genomic methods to estimate it. The resulting estimated breeding value (EBV) is a prediction of the animal’s true breeding value based on the information available at the time of evaluation.
- The relationship between true breeding value and EBV is therefore fundamental to genetic evaluation. The true breeding value is the underlying quantity of interest, while the EBV is the best available estimate of that quantity. As additional information becomes available, the EBV can change and generally becomes more reliable. New performance records, progeny records, relatives’ information, repeated measurements, pedigree information, and genomic data can all contribute to an updated estimate.
- The true breeding value is based specifically on additive genetic effects because these effects are transmitted from parents to offspring in a predictable statistical manner. If an animal has a favourable additive genetic value for growth, milk production, disease resistance, fertility, or another trait, part of that value is expected to be transmitted to its offspring. Dominance and epistatic effects can influence an animal’s own performance, but they are not transmitted from parent to offspring in the same predictable way as additive effects.
- The expected additive genetic value of an offspring can therefore be expressed as E(A_offspring) = (A_sire + A_dam) / 2, where A_sire and A_dam represent the breeding values of the sire and dam. This equation describes the expected genetic contribution of the parents. However, an individual offspring will not necessarily have exactly the average of its parents’ breeding values because of Mendelian sampling. During gamete formation, different offspring receive different combinations of parental alleles, creating genetic differences among siblings.
- Mendelian sampling is one reason why full siblings can have different breeding values even though they have the same parents. The parents determine the average genetic expectation for their offspring, but the particular combination of alleles inherited by each offspring determines its individual deviation from that expectation. This random component of inheritance is important in genetic evaluation and explains why the genetic merit of an individual cannot always be predicted perfectly from its parents alone.
- The true breeding value is also different from an animal’s total genotypic value. Genotypic value includes all genetic effects contributing to the phenotype, including additive, dominance, and epistatic effects. Breeding value focuses on the additive component. For breeding purposes, this distinction is essential because selection is intended to identify animals whose genetic superiority can be transmitted to future generations rather than animals whose performance depends primarily on non-additive interactions.
- Additive genetic variance provides the population-level variation in true breeding values. If a population has substantial additive genetic variance for a trait, animals differ genetically in ways that can respond to selection. The amount of additive genetic variation therefore determines an important part of the potential for genetic improvement. If additive genetic variance becomes very small, continued selection becomes increasingly difficult because genetically superior animals become harder to distinguish and the available response to selection declines.
- Narrow-sense heritability is closely associated with true breeding value because it measures the proportion of phenotypic variance attributable to additive genetic variance. It is expressed as h² = σ²_A / σ²_P, where σ²_A is additive genetic variance and σ²_P is phenotypic variance. High heritability generally means that phenotypic differences provide more information about differences in breeding value, whereas low heritability means that environmental variation contributes more strongly to observed differences.
- However, heritability should not be interpreted as the percentage of an individual animal’s phenotype that is genetic. Heritability is a population-level statistical parameter describing variation under particular conditions. A low-heritability trait can still have an important true breeding value and can respond to selection when sufficient genetic information is available. This is particularly important for fertility, disease resistance, survival, welfare, and other complex traits.
- The true breeding value cannot normally be measured directly because genes and their individual additive effects cannot simply be observed from an animal’s phenotype. Instead, breeders infer genetic merit from patterns of performance among the animal and its relatives. The greater the quantity and quality of relevant information, the more accurately the true breeding value can be predicted.
- An animal’s own phenotype can provide information about its true breeding value, particularly when the trait has moderate or high heritability and is measured accurately. For example, an animal with exceptional growth performance under standardized management may have a higher probability of having a favourable breeding value for growth than an animal with poor performance. However, the phenotype is never a perfect measure of true breeding value because environmental effects and measurement error are always present to some degree.
- Contemporary groups are therefore important when using individual performance records. Animals should be compared with appropriate contemporaries that have experienced similar environmental conditions, management, feeding, health status, housing, and other systematic influences. Proper adjustment for these effects helps prevent environmental advantages from being incorrectly interpreted as genetic superiority.
- Information from relatives can substantially improve prediction of true breeding value. Parents, full siblings, half siblings, grandparents, and other relatives share genes with the animal being evaluated. Their phenotypic records therefore contain information about the animal’s probable genetic merit. The usefulness of relative information depends on the degree of relationship, the number of relatives, the quality of their records, and the environmental conditions under which those records were obtained.
- Progeny testing provides particularly valuable information because offspring receive genes from the animal being evaluated. If a sire produces many offspring and those offspring consistently perform well for a particular trait under appropriate environmental conditions, this provides evidence that the sire has a favourable breeding value. Progeny testing can therefore improve the accuracy of breeding value estimation, although it traditionally requires substantial time and resources and can increase the generation interval.
- The use of pedigree information provides another way to predict true breeding value. Pedigree relationships allow genetic evaluation systems to account for the expected sharing of genes among relatives. The pedigree relationship matrix, commonly represented as the A matrix, describes expected additive relationships among animals. Combining pedigree relationships with phenotypic records allows statistical models to distinguish genetic differences from environmental differences.
- Modern breeding programmes increasingly use genomic information to improve prediction of true breeding value. Genomic markers provide information about the actual genetic relationships and chromosome segments carried by an animal. This information can be represented through a genomic relationship matrix, commonly known as the G matrix. Genomic data can therefore provide information that is more specific to the individual’s inherited genome than pedigree alone.
- Genomic selection uses genomic information to predict breeding values, producing genomic estimated breeding values (GEBVs). For young animals, genomic information can be particularly valuable because they may have little or no individual performance or progeny information. Genomic selection can therefore improve selection accuracy at an early age and allow breeders to select animals before they have produced offspring.
- The accuracy of an estimated breeding value can be described by the correlation between the estimated breeding value and the true breeding value. Higher selection accuracy means that the ranking of animals is more likely to reflect their actual genetic merit. Accuracy can be increased through larger and higher-quality datasets, more informative relatives, progeny records, repeated records, genetically correlated traits, improved statistical models, and genomic information.
- BLUP, or Best Linear Unbiased Prediction, is one of the major statistical methods used to estimate breeding values. BLUP uses information from multiple sources simultaneously and accounts for fixed environmental effects and relationships among animals. In an animal model, the general structure can be represented as y = Xb + Za + e, where y represents observations, b represents fixed effects, a represents additive genetic effects or breeding values, and e represents residual effects. The model allows the available data to be used to predict the genetic merit of individual animals.
- True breeding value is also central to multi-trait genetic evaluation. Many economically and biologically important traits are genetically correlated. For example, growth, carcass composition, feed efficiency, fertility, disease resistance, and survival may have genetic relationships that affect selection outcomes. When traits are genetically correlated, information from one trait can improve prediction of breeding value for another trait, especially when the second trait is difficult or expensive to measure.
- The additive genetic relationship between two traits can be expressed as r_A = Cov_A(X,Y) / (σ_A,X × σ_A,Y), where Cov_A(X,Y) is the additive genetic covariance between traits and σ_A,X and σ_A,Y are their additive genetic standard deviations. A favourable genetic correlation can allow improvement in one trait to produce a favourable correlated response in another trait, while an unfavourable correlation can create a genetic trade-off. This is why true breeding values are often evaluated for several traits simultaneously rather than treating every trait independently.
- True breeding values are particularly important when breeding programmes use a selection index. A selection index combines information from several traits or information sources into a single ranking criterion. A general index can be written as I = b₁x₁ + b₂x₂ + … + bₙxₙ, where x values represent selection information and b values represent the coefficients assigned to those sources. The index is designed to identify animals with high expected aggregate genetic merit according to the breeding objective.
- A breeding objective can be represented as H = a₁A₁ + a₂A₂ + … + aₙAₙ, where A values are true breeding values for the traits included in the objective and a values represent their relative importance. In practical breeding programmes, the true breeding values are unknown, so estimated breeding values are used to approximate the genetic merit described by the breeding objective. This framework allows breeders to balance production, fertility, health, disease resistance, feed efficiency, survival, welfare, adaptation, and other economically or biologically important traits.
- True breeding value is therefore not synonymous with phenotypic superiority. Selecting animals solely because they have the highest observed performance can be misleading if environmental effects are substantial. A highly fed animal may grow rapidly because of superior nutrition rather than superior genetics, while an animal exposed to poor conditions may have an excellent true breeding value despite a relatively modest phenotype. Genetic evaluation attempts to identify the genetic component behind the observed performance.
- The same principle applies to health and disease-related traits. An animal that remains healthy may have favourable genetic resistance, but its observed health status may also reflect vaccination, exposure, management, nutrition, housing, and disease pressure. Therefore, accurate estimation of breeding value for disease resistance or health traits requires appropriate environmental adjustment and sufficient records across animals and environments.
- For fertility and reproductive traits, the distinction between phenotype and breeding value can be even more important because environmental and management factors can strongly influence reproductive performance. Age, nutrition, disease, reproductive management, season, and housing can all affect fertility. Genetic evaluation can help separate these effects and identify animals with favourable additive genetic merit for reproductive performance.
- The same principle applies to animal welfare, temperament, survival, longevity, heat tolerance, stress resistance, and adaptation. These traits are often influenced strongly by environmental conditions, and some may have relatively low heritability. Nevertheless, they can possess meaningful additive genetic variation and can be incorporated into breeding programmes when accurate phenotypic and genomic information is available.
- True breeding value also has an important relationship with genetic gain. The rate of genetic improvement depends on selection intensity, selection accuracy, additive genetic variation, and generation interval. A commonly used expression is ΔG/year = i × r × σ_A / L, where i represents selection intensity, r represents accuracy of selection, σ_A represents additive genetic standard deviation, and L represents generation interval. More accurate prediction of true breeding value increases the probability that selected animals genuinely have superior genetic merit.
- Reducing the generation interval can also increase the rate of genetic improvement. This is one reason genomic selection has become important in many breeding programmes. If young animals can be evaluated accurately using genomic information, breeders may be able to select them earlier rather than waiting for extensive progeny information. However, the benefits depend on the reliability of genomic predictions and the quality and relevance of the reference population.
- True breeding value should also be considered in relation to genetic diversity. Selecting only the animals with the highest breeding values can result in excessive use of a small number of individuals, particularly elite sires. This can increase genetic concentration, relatedness, and the rate of inbreeding. Therefore, breeding programmes should balance genetic gain with the maintenance of sufficient genetic diversity.
- The expected inbreeding of offspring depends on the genetic relationship between their parents. It can be expressed as E(F_offspring) = φ(sire, dam), where φ represents the coefficient of coancestry between the sire and dam. Under the usual relationship framework, the additive relationship between two individuals is approximately r ≈ 2φ. These relationships can be incorporated into mate allocation and optimal contribution selection to prevent excessive concentration of genetic contributions.
- The effective population size is also relevant because it influences the expected rate of inbreeding. Under simplified assumptions, the change in inbreeding per generation can be approximated as ΔF ≈ 1 / (2Ne), where Ne is the effective population size. A smaller effective population size generally results in faster accumulation of inbreeding and loss of genetic diversity. Consequently, selection based on true or estimated breeding value should be accompanied by appropriate population management.
- Genomic information can provide additional insight into genetic diversity through runs of homozygosity (ROH). Long ROH segments can indicate inheritance of identical-by-descent chromosome segments from common ancestors. A genomic measure of homozygosity can be summarized as F_ROH = Total length of ROH / Total autosomal genome length. This information can complement pedigree-based measures of relatedness and inbreeding when managing breeding populations.
- True breeding value is also relevant to the management of deleterious genetic variants. Recessive harmful alleles can remain hidden in heterozygous animals and may become expressed when two carriers are mated. Genetic testing, genomic information, pedigree relationships, and careful mate allocation can help reduce the probability of producing affected offspring without unnecessarily removing all carriers from a population and thereby reducing genetic diversity.
- Another important consideration is genotype–environment interaction (G×E). The true breeding value of an animal is defined relative to a trait and evaluation framework, but the expression and ranking of genetic merit can change across environments. An animal that performs exceptionally well under temperate conditions may not have the same relative genetic advantage under heat stress, disease challenge, nutritional limitation, or another production environment. For climate adaptation and resilience traits, breeding programmes may therefore need data from the environments in which animals are expected to perform.
- The reliability of true breeding value prediction depends heavily on the quality of the data used for evaluation. Accurate phenotypes, consistent trait definitions, reliable pedigree records, appropriate contemporary groups, sufficient sample sizes, and suitable statistical models are essential. Genomic prediction additionally depends on a relevant reference population containing animals with both reliable phenotypes and genomic information.
- It is important to remember that a true breeding value is not a permanent observable characteristic such as body weight or height. It is a statistical genetic quantity associated with the animal’s additive genetic constitution and a defined trait and population. An animal’s EBV can change as additional information becomes available, while the underlying true breeding value remains a theoretical quantity that the evaluation system is attempting to predict.
- The distinction between true breeding value and EBV is therefore particularly important for interpreting genetic evaluations. True breeding value represents the actual additive genetic merit, whereas EBV represents an estimate based on available information. A high EBV indicates that the evaluation system predicts a high true breeding value, but the prediction is not perfect. The reliability or accuracy associated with the EBV indicates how much confidence should be placed in that prediction.
- Young animals often have less information and therefore lower EBV reliability than older animals with extensive performance or progeny information. Genomic selection can partially overcome this limitation by providing additional information about the individual’s genome. As the animal accumulates records and progeny information, its EBV may become more accurate and may change accordingly.
- The practical importance of true breeding value lies in its role as the ultimate genetic target of selection. Breeders are not fundamentally interested in selecting animals simply because they have high phenotypic measurements; they are interested in identifying animals whose genetic superiority will contribute favourably to future generations. True breeding value represents that underlying additive genetic merit, while EBVs, GEBVs, BLUP, progeny testing, genomic selection, and selection indexes provide practical tools for predicting and using it.
- A successful animal breeding programme therefore combines accurate prediction of breeding values with appropriate breeding objectives, selection criteria, genetic diversity management, and long-term population planning. True breeding value provides the theoretical foundation, while modern genetic evaluation methods transform that theory into practical selection decisions. By combining phenotypic records, pedigree relationships, family information, progeny performance, multi-trait information, and genomic data, breeders can improve the accuracy of genetic selection and increase the rate of desirable genetic change.
- Ultimately, true breeding value is the genetic quantity that explains an animal’s expected additive contribution to future generations. Understanding it helps breeders distinguish genetic merit from environmental performance, understand why EBVs are estimates rather than direct measurements, interpret selection accuracy, and appreciate the role of BLUP and genomic selection. It also provides the foundation for balancing genetic gain with fertility, health, survival, welfare, adaptation, and genetic diversity, making it one of the central concepts in modern quantitative genetics and sustainable animal breeding.
