- The relationship between animals is a fundamental concept in animal breeding, population genetics, and quantitative genetics. It describes the extent to which two animals are genetically connected through common ancestry and therefore are expected to share genes inherited from common ancestors. Understanding genetic relationships is essential for estimating breeding values, predicting inbreeding, evaluating relatives, designing mating plans, and managing genetic diversity in breeding populations.
- Animals can be related because they share parents, grandparents, more distant ancestors, or because they belong to the same population and have inherited genetic material from common ancestors over many generations. The degree of relationship generally decreases as the number of generations separating two animals increases, although the actual genetic sharing between individuals is influenced by Mendelian sampling, recombination, population history, and random inheritance.
- In animal breeding, it is important to distinguish the biological idea of relatedness from the statistical measures used to quantify it. Two animals may be described as relatives because they have common ancestors, while a relationship coefficient, additive relationship, or genomic relationship provides a numerical representation of their expected or observed genetic similarity under a particular model.
- The classical pedigree-based concept of relationship is based on the probability that two animals share alleles that are identical by descent (IBD). Alleles are identical by descent when they originated from the same ancestral allele. This concept is different from identity by state, where two alleles have the same DNA sequence but may have arisen independently or from different ancestral copies.
- The relationship between animals is closely connected to the inbreeding coefficient, but these two concepts describe different things. The relationship coefficient describes the genetic relationship between two individuals, whereas the inbreeding coefficient describes the probability that the two alleles inherited by one individual at a locus are identical by descent. Consequently, the relationship between a prospective sire and dam can be used to predict the expected inbreeding of their offspring.
- For example, a parent and its offspring are closely related because the offspring receives approximately half of its nuclear genetic material from that parent. Full siblings are also closely related because they have the same parents, although the exact amount of DNA they inherit from those parents varies because of Mendelian sampling and recombination. Half-siblings share one parent and are therefore expected to be less closely related than full siblings.
- Grandparent-grandchild relationships, uncle-niece relationships, half-sibling relationships, and relationships between more distant relatives can also be quantified through pedigree analysis. As common ancestors become more distant, the expected proportion of alleles shared through those ancestors generally decreases. However, animals can have multiple common ancestors, so the total relationship can be greater than would be expected from a single ancestral pathway.
- The pedigree is one of the traditional tools for determining relationships among animals. A pedigree records the parents and ancestors of each individual and allows relationships to be traced through generations. By following the paths connecting two animals to their common ancestors, breeders can estimate their expected genetic relationship.
- Pedigree information can be represented mathematically using the numerator relationship matrix, commonly called the additive relationship matrix or A matrix. This matrix contains the expected additive genetic relationships among all animals included in the pedigree. Each element represents the expected proportion of genes that two animals share because of common ancestry under the assumptions of the pedigree model.
- The diagonal elements of the additive relationship matrix are also related to individual inbreeding. For an animal with pedigree inbreeding coefficient FF, the diagonal relationship is commonly expressed as 1+F1+F. Off-diagonal elements describe the expected additive relationship between different animals. This matrix is fundamental to many statistical methods used in animal breeding.
- The concept of additive genetic relationship is particularly important because additive genetic effects are transmitted from parents to offspring and form the basis of prediction of breeding value. Breeding values describe an animal’s expected genetic contribution to future offspring for a particular trait. Relatives provide information about one another because they are expected to share some genetic effects.
- For example, an animal with limited own performance records may still receive a useful breeding-value estimate if its parents, siblings, offspring, or other relatives have informative records. The relationship between the animals determines how much information from one individual’s phenotype contributes to the evaluation of another animal.
- This principle is central to BLUP (Best Linear Unbiased Prediction). Animal-model BLUP uses information from an animal’s own performance and the performance of relatives, together with the pedigree relationship structure among animals. The relationship matrix allows the statistical model to account for the fact that records from close relatives contain more information about an individual’s genetic merit than records from unrelated animals.
- The relationship between animals therefore plays a major role in genetic evaluation. An animal does not need to have a direct measurement for every trait to receive a breeding-value estimate. Information can flow through the pedigree because genetically related animals share expected genetic effects. This is particularly valuable for traits that are difficult, expensive, late in life, sex-limited, or impossible to measure directly in selection candidates.
- The relationship between animals also affects the accuracy of breeding value prediction. When several closely related animals have reliable phenotypic records, those records can provide substantial information about the genetic merit of their relatives. However, the information is not completely independent because relatives share genes. Statistical genetic models account for these relationships when estimating breeding values.
- An important distinction is between expected genetic relationship and realized genetic sharing. Pedigree relationships describe expectations based on ancestry. Because inheritance is random, two animals with the same pedigree relationship may not share exactly the expected proportion of their genome. This difference is particularly important for full siblings, who have the same parents but do not inherit exactly the same chromosome segments from those parents.
- This phenomenon is partly explained by Mendelian sampling. Each offspring receives a random sample of parental alleles, and recombination creates different combinations of chromosome segments in different offspring. Consequently, full siblings can differ substantially in their actual genomic similarity even though their pedigree relationship is the same.
- This limitation of pedigree relationships has encouraged the use of genomic relationship information. Genomic methods use DNA markers distributed across the genome to estimate the realized genetic similarity between animals. Rather than asking only whether two animals have common ancestors, genomic relationship analysis can examine how much genetic material they actually share at the measured markers.
- A genomic relationship matrix, often called the G matrix, can therefore provide a more detailed representation of genetic similarity than a pedigree-based relationship matrix. Genomic relationships can identify animals that are more or less related than their pedigree would suggest, particularly when pedigrees are incomplete or when animals have the same recorded ancestors but differ in their realized inheritance.
- Genomic relationship information is especially valuable in populations with deep genomic data and accurate genotyping. It can improve the accuracy of genomic selection, help verify parentage, identify unexpected relationships, monitor genetic diversity, and improve mating decisions.
- Genomic relationship is not exactly the same as pedigree relationship. Pedigree relationship is an expectation based on recorded ancestry, whereas genomic relationship is estimated from observed marker data and depends on the genomic model, marker panel, allele frequencies, reference population, and statistical method used. Therefore, the numerical values from different relationship systems should not be treated as automatically interchangeable.
- The relationship between animals can also be affected by the reference population used in genomic analysis. Genomic relationship estimates depend partly on allele-frequency assumptions and the population against which relationships are measured. This means that genomic relationship values must be interpreted within the context of the population and analytical method.
- The relationship between animals is particularly important when managing inbreeding. If two prospective parents are closely related, their offspring have a higher expected probability of receiving identical-by-descent alleles from common ancestors. Therefore, the relationship between the sire and dam can be used to calculate or predict the expected inbreeding coefficient of their offspring.
- A breeder who selects parents independently based only on their individual breeding values may unintentionally mate highly related animals. For example, the same popular sire may be used extensively across a population. Over several generations, many animals may become descendants of that sire, increasing the probability that apparently different breeding candidates share common ancestry.
- The popular sire effect is therefore closely connected with animal relationships. A sire that produces a very large number of offspring can make a substantial genetic contribution to the population. As his descendants become common, the average relationship among animals in the population may increase. This can contribute to an increase in the population’s rate of inbreeding even when breeders do not intentionally mate close relatives.
- The relationship between animals is also influenced by effective population size. A population with a small effective population size tends to accumulate genetic relatedness more rapidly because relatively few individuals contribute genes to subsequent generations. Unequal reproductive success, small numbers of breeding males, unequal family sizes, and intensive use of particular animals can all reduce effective population size and increase relatedness.
- Genetic drift can further influence relationships within populations. In finite populations, random changes in allele frequencies can cause certain ancestral lineages to become more common while others disappear. Over generations, surviving animals may increasingly descend from the same limited group of ancestors. This can increase genetic relatedness even without deliberate close-relative mating.
- Relationship analysis is therefore important not only for individual mating decisions but also for population management. Breeders can examine the average relationship among animals, the contribution of different families, the number of descendants produced by important sires and dams, and changes in relationship across generations.
- One of the most important practical applications is mate allocation. Mate allocation involves choosing specific male-female combinations rather than simply selecting parents independently. A breeding program can prioritize animals with high breeding values while avoiding pairings that would produce excessive expected inbreeding.
- For example, two males may have similar breeding values for the breeding objective, but one may be closely related to many females in the population while the other is less related to them. The second male may therefore provide greater opportunities to achieve genetic improvement without increasing inbreeding as rapidly.
- More advanced programs use optimal contribution selection. Instead of simply deciding which animals should be selected, optimal contribution methods determine the appropriate genetic contribution of each selected animal to the next generation. This approach can control the increase in average relationship and inbreeding while maintaining genetic gain.
- The relationship between animals is also important in conservation breeding. Small or endangered populations often contain limited genetic diversity, and breeding decisions must carefully balance reproductive success with the preservation of genetic variation. Selecting mating pairs that minimize excessive relatedness can help slow the accumulation of inbreeding.
- In conservation populations, maintaining multiple ancestral lineages can be particularly important. If a small number of animals contribute most of the next generation, some genetic lineages may become underrepresented or disappear. Monitoring relationships can therefore help maintain a more balanced distribution of genetic contributions.
- Relationship analysis is also relevant to crossbreeding. Animals from different breeds or genetically distinct populations may have lower average within-population relatedness, although their relationship depends on their evolutionary and breeding histories. Crossing genetically distinct populations can increase heterozygosity and may produce heterosis, or hybrid vigor, for certain traits.
- However, lower relationship between breeds does not automatically mean that crossbreeding is always superior. Breed complementarity, adaptation, production environment, maternal effects, reproductive performance, health, and the long-term breeding objective must also be considered. Relationship is one component of a broader breeding strategy.
- The relationship between animals can also be used to understand Mendelian sampling and genetic differences among relatives. Although relatives share expected genetic material, they are not genetically identical except in special cases such as genetically identical individuals. Even close relatives can differ substantially in breeding value because of the random inheritance of chromosome segments.
- This is why selection among siblings can be effective. Full siblings share parents but inherit different combinations of parental alleles. Some may receive more favorable combinations for a particular trait than others. Genomic information can help identify these differences and improve selection accuracy.
- The relationship between animals is also important when interpreting phenotypic records. If several related animals are raised in the same environment, their phenotypes may resemble each other because of both shared genes and shared environmental effects. A genetic evaluation must therefore distinguish genetic similarity from common environmental effects.
- For example, littermates may experience the same maternal environment, prenatal conditions, nutrition, housing, and management. Their similar performance may therefore result from genetic relationship, shared environment, maternal effects, or combinations of these factors. Statistical models account for such effects when sufficient data are available.
- This distinction is particularly important in young animals. Their early-life performance can be influenced strongly by maternal effects, including maternal genotype, uterine environment, milk production, maternal behaviour, and early nutrition. Consequently, the observed similarity between related animals is not necessarily entirely genetic.
- The relationship between animals is also relevant to the estimation of heritability and genetic variance. Quantitative genetic analyses use patterns of resemblance among relatives to separate genetic and environmental sources of variation. Parents, offspring, siblings, half-siblings, and other relatives provide different types of information about genetic variation.
- For example, the resemblance between relatives can provide evidence about the contribution of additive genetic variance to phenotypic differences. However, accurate estimation requires appropriate statistical models because relatives may also share environments, maternal effects, management conditions, and other sources of covariance.
- The relationship between animals can therefore be viewed as a central connection between pedigree, genetic evaluation, breeding value, heritability, inbreeding, and genetic diversity. These concepts are not isolated. They form an interconnected framework for understanding how genes are transmitted through breeding populations.
- In modern breeding programs, relationship information can be combined with phenotypic records, pedigree data, genomic information, and breeding objectives. This creates a more complete genetic evaluation system in which both genetic merit and genetic relationships are considered simultaneously.
- A particularly important development is the integration of genomic relationship into single-step genomic evaluation. Such approaches combine pedigree, phenotypic, and genomic information within a unified genetic evaluation framework. This allows animals with genomic data to contribute information to evaluations while maintaining connections with animals that have pedigree and performance records.
- Relationship information can also help identify parentage errors. If the recorded sire or dam is genetically inconsistent with the offspring, genomic analysis may reveal that the recorded pedigree is incorrect. Correcting parentage can improve relationship estimates, breeding-value prediction, inbreeding calculations, and selection decisions.
- Accurate relationship information is therefore dependent on accurate data. Errors in pedigree records, missing ancestors, incorrect parent assignments, or insufficient genomic information can lead to misleading estimates. Breeding programs should maintain high-quality identification, pedigree, genotype, and performance records.
- Another important concept is coancestry. Coancestry, or kinship, describes the probability that an allele randomly selected from one individual and an allele randomly selected from another individual are identical by descent. It is closely related to the relationship coefficient and is particularly useful for understanding the expected inbreeding of offspring resulting from a particular mating.
- If two animals have high coancestry, mating them is more likely to produce offspring with higher expected inbreeding. This is why relationship and coancestry measures are frequently incorporated into mating programs.
- The relationship between animals can also be represented as a relationship matrix. Each row and column represents an animal, and each cell represents the estimated relationship between a particular pair. Such matrices allow breeders and statistical models to consider relationships across an entire population rather than evaluating animals one pair at a time.
- In a large breeding population, relationship matrices can contain information for thousands or millions of animal pairs. Computational methods are therefore essential for modern genetic evaluation. Genomic data can make these analyses even more information-rich but also computationally demanding.
- The biological meaning of a relationship coefficient should always be considered alongside the statistical definition used to calculate it. Different relationship measures can have different scales and interpretations. Additive relationship, pedigree relationship, coancestry, and genomic relationship are related concepts but should not be treated as identical.
- The same principle applies when comparing relationship values between populations. A relationship measure calculated within a closed breed may have a different interpretation from one calculated across several breeds. Population structure, allele frequencies, founder definitions, and reference populations all influence the numerical values.
- Relationship between animals is therefore best understood as a probabilistic concept rather than a simple statement such as “these animals share exactly X% of their genes.” Expected relationship values describe averages across inheritance pathways or populations. Individual animals can deviate from these expectations because of random segregation and recombination.
- For example, two full siblings have the same parents and therefore have the same expected pedigree relationship. Nevertheless, they can differ considerably in the actual chromosome segments inherited from their parents. This difference explains why genomic relationship can provide additional information beyond pedigree relationship.
- The practical importance of relationship information extends across nearly every area of animal breeding. It supports breeding-value estimation, BLUP, genomic selection, inbreeding prediction, mate allocation, optimal contribution selection, genetic diversity management, conservation breeding, parentage verification, and population genetic analysis.
- Relationship information also helps breeders balance short-term and long-term objectives. Selecting animals with high genetic merit can increase genetic gain, but repeatedly using closely related animals can increase inbreeding and reduce genetic diversity. By incorporating relationship information into selection and mating decisions, breeders can pursue genetic improvement while controlling the accumulation of relatedness.
- The goal is therefore not necessarily to select animals that are unrelated to everyone else. In a closed or intensively selected population, some degree of relatedness is unavoidable. The objective is to manage the distribution of genetic contributions so that valuable genetic improvement can continue without excessive loss of diversity.
- In summary, the relationship between animals describes their genetic connection through common ancestry and shared inherited genetic material. Pedigree relationships provide expected genetic similarity based on ancestry, while genomic relationships estimate realized genetic similarity using DNA information. Relationship is closely connected with coancestry, identity by descent, inbreeding coefficient, breeding value, effective population size, and genetic diversity.
- Understanding relationships among animals is essential for modern animal breeding because relatives provide information about genetic merit, while excessive relatedness can increase the risk of inbreeding. Pedigree analysis, relationship matrices, genomic relationship, BLUP, genomic selection, and mate allocation allow breeders to use relationship information strategically.
- Ultimately, successful breeding programs must consider both who is genetically superior and how animals are related to one another. Combining genetic merit with relationship information makes it possible to achieve sustainable genetic gain, control inbreeding, preserve genetic diversity, and maintain healthy and productive breeding populations across generations.
