- Pedigree-based relatedness describes the expected genetic relationship between animals based on their recorded ancestry. It is one of the fundamental concepts in animal breeding, quantitative genetics, and population genetics because it allows breeders and geneticists to determine how closely animals are related, predict the expected genetic similarity between individuals, estimate the risk of inbreeding, and manage genetic diversity within a population.
- A pedigree records the parents, grandparents, and other ancestors of animals across generations. By tracing these ancestral connections, it is possible to estimate how much genetic material two animals are expected to share because of their common ancestry. Pedigree-based relatedness therefore represents an expected relationship, rather than a measurement of the exact DNA segments actually inherited by two animals.
- The concept is closely connected to kinship, coancestry, coefficient of relationship, inbreeding coefficient, identity by descent, and the additive relationship matrix. These concepts are related but are not interchangeable. Understanding their differences is important when interpreting genetic relationships and making breeding decisions.
- At the simplest level, two animals are genetically related when they have one or more common ancestors. For example, a parent and offspring are related because the offspring inherits genes from the parent. Full siblings are related because they receive genes from the same parents. Half-siblings share one parent, while first cousins share grandparents through their respective parents. The further back the common ancestors occur, the lower the expected relationship generally becomes.
- Pedigree-based relatedness is based on the principle of identity by descent (IBD). Two copies of an allele are considered identical by descent when they originate from the same ancestral copy through inheritance. A pedigree does not normally identify the exact DNA sequence inherited at every locus. Instead, it uses the known ancestry to calculate the expected probability that alleles are inherited from common ancestors.
- The coefficient of relationship is commonly used to express the expected proportion of additive genetic material shared by two individuals because of common ancestry. Under the conventional diploid definition, the relationship coefficient is twice the kinship coefficient.
- A commonly used relationship between the two measures is:
- r(i,j) = 2 × φ(i,j)
- where r(i,j) is the coefficient of relationship between animals i and j, and φ(i,j) is their kinship or coancestry coefficient.
- For example, under standard assumptions and with non-inbred animals, the expected coefficient of relationship between a parent and offspring is 0.50. Full siblings also have an expected relationship of 0.50, while half-siblings have an expected relationship of 0.25.
- Typical pedigree-based expectations include approximately 0.50 for parent-offspring, 0.50 for full siblings, 0.25 for half-siblings, 0.25 for grandparent-grandchild, 0.25 for aunt or uncle and niece or nephew, and 0.125 for first cousins. Two unrelated animals may be assigned an expected relationship of approximately zero when no known common ancestry exists within the pedigree.
- These values are expectations rather than guarantees. Two full siblings do not necessarily inherit exactly the same proportion of their parents’ genomes. During meiosis, chromosomes undergo segregation and recombination, meaning that each offspring receives a different combination of parental genetic material. Consequently, the actual DNA shared by two relatives can differ substantially from the simple pedigree expectation.
- This distinction between expected relatedness and realized genetic sharing is extremely important. Pedigree-based relatedness describes what is expected from ancestry, whereas genomic methods can estimate the genetic relationship actually observed across the genome.
- Pedigree-based relatedness begins with the quality of the pedigree itself. A pedigree normally contains information about an animal, its sire, its dam, and potentially multiple generations of ancestors. The deeper and more accurate the pedigree, the more information is available for calculating relationships among animals.
- Consider two animals that share the same sire but have different dams. They are expected to be half-siblings and therefore have an expected relationship coefficient of approximately 0.25 when the parents are unrelated and non-inbred. If they share both sire and dam, they are full siblings and their expected relationship is approximately 0.50.
- The presence of common ancestors can create relationships through multiple ancestral pathways. For example, two animals may share several ancestors because their pedigrees contain repeated individuals. In such cases, their overall pedigree relationship is obtained by considering the contributions from all relevant ancestral pathways rather than considering only one common ancestor.
- This becomes especially important in populations where inbreeding has accumulated over multiple generations. If ancestors themselves are related, the relationship between their descendants can be higher than the simple textbook value expected for a particular family relationship.
- Pedigree-based relatedness is therefore closely connected to the inbreeding coefficient. The inbreeding coefficient of an individual represents the probability that the two alleles at a random locus are identical by descent because of common ancestry. Pedigree relatedness, by contrast, describes the expected genetic connection between two different individuals.
- Under conventional definitions, the expected inbreeding coefficient of offspring can be predicted from the kinship between their parents:
- E(F_offspring) = φ(sire, dam)
- Because the conventional coefficient of relationship is twice kinship, this can also be expressed as:
- E(F_offspring) = r(sire, dam) / 2
- This relationship is extremely useful in breeding management. If a proposed sire and dam are closely related, their offspring are expected to have a higher level of inbreeding. Breeders can therefore use pedigree relationships to avoid mating combinations that would create excessive relatedness or inbreeding.
- The additive relationship matrix, commonly called the A matrix, provides a systematic way to represent pedigree-based relationships among all animals in a population. Each element of the matrix represents the expected additive genetic relationship between a pair of animals.
- For example, an A matrix for a small population might contain information indicating that an animal has a relationship of approximately 0.50 with its parent, approximately 0.50 with a full sibling, and approximately 0.25 with a half-sibling, depending on the pedigree and inbreeding status.
- The diagonal elements of the additive relationship matrix also contain important information. For an individual, the diagonal element is related to its expected additive genetic relationship with itself and incorporates its inbreeding coefficient. Under the conventional pedigree relationship definition:
- A(i,i) = 1 + F(i)
- where A(i,i) is the diagonal element of the additive relationship matrix and F(i) is the inbreeding coefficient.
- The off-diagonal elements describe the expected additive relationship between different animals. These relationships form the basis of many statistical models used in genetic evaluation.
- Pedigree-based relatedness is particularly important in the animal model used for estimating breeding values. Genetic evaluation systems use information from an animal’s own performance, relatives’ performance, pedigree relationships, and other sources of information to estimate estimated breeding values (EBVs).
- The relationship matrix allows information from relatives to contribute appropriately to the genetic evaluation of an animal. If an animal has no own performance record for a particular trait, information from its parents, siblings, offspring, and other relatives can still contribute to its estimated genetic merit.
- This is one of the foundations of BLUP (Best Linear Unbiased Prediction) in animal breeding. BLUP models use pedigree relationships to account for the expected sharing of additive genetic effects among animals.
- Pedigree-based relationships are also important for estimating genetic variance and heritability. When records from related animals are available, the covariance among relatives provides information about the amount of variation attributable to genetic differences.
- However, genetic evaluation must distinguish genetic relationships from shared environmental effects. For example, full siblings raised in the same pen may resemble each other because they share genes, but they may also resemble each other because they experience the same nutrition, housing, management, maternal environment, or disease exposure. Statistical models therefore need to account for common environmental effects, maternal effects, and other sources of resemblance where appropriate.
- Pedigree-based relatedness is also useful for selection decisions. Breeders do not select animals solely on their individual performance. They may consider EBVs, genetic relationships, inbreeding risk, genetic diversity, and the contribution an animal would make to the next generation.
- Suppose two candidate sires have similar breeding values for an economically important trait. If one sire is closely related to a large proportion of the breeding females, using that sire extensively could increase future inbreeding. The other sire may have similar genetic merit but provide greater genetic diversity. Pedigree-based relatedness can therefore contribute to a more balanced breeding decision.
- This issue is particularly important with the popular sire effect. When a small number of genetically successful males produce a very large proportion of offspring, their genes become widely distributed throughout the population. As their descendants are subsequently selected for breeding, relationships among animals can increase rapidly. Over generations, this can increase the rate of inbreeding and reduce genetic diversity.
- Pedigree-based relatedness is therefore useful for monitoring and controlling the rate of inbreeding. Under simplified population assumptions, the expected increase in inbreeding per generation can be approximated by:
- ΔF ≈ 1 / (2Ne)
- where ΔF is the expected increase in inbreeding per generation and Ne is the effective population size.
- Although this approximation is useful for understanding the relationship between effective population size and inbreeding, real breeding populations may deviate from the assumptions because of unequal reproductive success, selection, overlapping generations, population structure, sex ratio differences, and other factors.
- Pedigree information is especially valuable for mate allocation. In mate allocation, breeding pairs are selected while considering their genetic merit and their relationship with each other. The objective may be to produce genetically superior offspring while limiting the expected increase in inbreeding.
- More sophisticated breeding programs may use optimal contribution selection, in which the contribution of individual animals to the next generation is optimized rather than simply selecting the animals with the highest breeding values. Pedigree relationships are an important component of these calculations because they allow the expected genetic consequences of reproductive contributions to be evaluated.
- Pedigree-based relatedness is also important for conservation breeding. Small or endangered populations can experience rapid increases in relatedness and inbreeding because the number of available breeding animals is limited. Maintaining genetic diversity requires breeders to understand which individuals are related and how reproductive contributions should be distributed.
- In conservation programs, mean kinship can be used to identify animals that represent relatively rare portions of the population’s genetic diversity. Such information can help prevent the repeated use of only a few popular families and support long-term maintenance of genetic variation.
- Pedigree-based relatedness is also relevant to crossbreeding. When animals from genetically distinct breeds are crossed, their pedigree relationship within the population may be low or effectively zero under the relevant reference population. Crossbreeding can therefore reduce the probability of close inbreeding within the crossbred population and can produce heterosis, also called hybrid vigor, for some traits.
- However, pedigree-based relatedness depends strongly on the population and pedigree definition being used. Two animals described as unrelated may still share genetic segments if their common ancestry occurred before the available pedigree began. Therefore, an apparent relationship of zero does not necessarily mean that the animals have no shared DNA.
- This is one of the major limitations of pedigree-based relatedness. A pedigree can only use ancestry that has been recorded or otherwise known. If pedigree records are incomplete, incorrect, or shallow, the calculated relationship may not accurately reflect the animals’ actual genomic relationship.
- Pedigree errors can occur when parentage is incorrectly recorded, animals are accidentally assigned to the wrong parents, or historical records are incomplete. Even a relatively small number of incorrect parent assignments can affect relationship calculations and genetic evaluations.
- The number of recorded generations also matters. If a pedigree begins with a set of founders whose parents are unknown, the relationship among those founders is often assumed to be zero under conventional pedigree calculations. In reality, the founders may have shared ancestry that is simply not recorded.
- This is known as the founder assumption and is an important reason why pedigree-based and genomic estimates of relatedness can differ.
- Modern animal breeding increasingly combines pedigree information with genomic information. Genomic data can be obtained using SNP genotyping or other DNA-based technologies and can be used to estimate realized relationships across the genome.
- A genomic relationship matrix, commonly called the G matrix, represents relationships estimated from genetic marker information. In contrast, the A matrix represents expected relationships derived from pedigree.
- The two matrices answer related but somewhat different questions. The pedigree relationship matrix describes the expected relationship based on known ancestry, while the genomic relationship matrix provides information about the genetic similarity actually observed at measured genomic markers.
- For example, two full siblings have an expected pedigree relationship of approximately 0.50, but their realized genomic relationship may be somewhat higher or lower because they inherit different chromosome segments from their parents. Genomic data can capture this variation.
- Genomic information can also reveal cryptic relatedness, where animals appear unrelated from the pedigree but share genomic segments because of unrecorded common ancestry. This is particularly valuable in populations with incomplete pedigrees.
- Another important application is the identification of runs of homozygosity (ROH). Long stretches of homozygous DNA can indicate relatively recent shared ancestry, while shorter ROH can reflect more distant ancestry, although interpretation depends on population history, marker density, and analytical methods.
- Combining pedigree and genomic information can improve the accuracy of genetic relationship estimates. This is especially useful for genomic selection, where genomic information is incorporated into the prediction of breeding values.
- Pedigree-based relatedness also supports parentage verification. If the recorded sire and dam are inconsistent with the genetic relationships observed among animals, genomic testing can help identify incorrect parentage. Accurate parentage improves the quality of pedigrees and therefore improves subsequent relationship calculations and genetic evaluations.
- Pedigree-based relatedness is also important when analyzing genetic diversity. A population with many animals is not necessarily genetically diverse if most animals descend from a small number of highly used ancestors. Pedigree analysis can reveal the concentration of ancestry and identify families that are overrepresented.
- This is particularly relevant when selection is intense. Strong selection can increase the frequency of desirable alleles but may also reduce the number of animals contributing genes to future generations. When combined with unequal reproductive success, this can increase relatedness and the rate of inbreeding.
- For this reason, modern breeding programs aim to balance genetic gain with genetic diversity. The objective is not simply to minimize relatedness or inbreeding at all costs. Excessive restriction of relatedness can reduce selection response, while ignoring relatedness can lead to rapid accumulation of inbreeding. A balanced breeding strategy considers both genetic merit and population management.
- Pedigree-based relatedness can also be incorporated into selection indexes. A selection index can combine breeding values for multiple traits with economic weights and, where appropriate, constraints or penalties associated with inbreeding and genetic diversity.
- For example, a breeding program may simultaneously consider milk production, fertility, disease resistance, longevity, conformation, temperament, and inbreeding risk. Pedigree relationships help determine how selected animals will influence the genetic structure of future generations.
- One common mistake is to interpret a relationship coefficient of 0.50 as meaning that two animals share exactly 50% of their DNA. This is not correct. A relationship of 0.50 represents an expected additive genetic relationship under the relevant assumptions. The actual genome-wide sharing between relatives varies because of Mendelian segregation and recombination.
- Another common mistake is to confuse coefficient of relationship with kinship coefficient. Under conventional diploid definitions:
- r(i,j) = 2 × φ(i,j)
- Therefore, a parent-offspring pair has an expected relationship of approximately 0.50 and an expected kinship of approximately 0.25.
- A third common mistake is to confuse the inbreeding coefficient with the relationship between two animals. Inbreeding is a within-individual concept, while relationship and kinship generally describe genetic connections between individuals. The relationship between a prospective sire and dam is relevant because it helps predict the expected inbreeding of their offspring.
- Pedigree-based relatedness should also not be confused with phenotypic similarity. Two animals may look alike because they share genes, but they can also resemble each other because they were raised in the same environment. Conversely, genetically related animals may look quite different because of environmental influences, random genetic effects, age, sex, nutrition, disease, and management.
- The usefulness of pedigree-based relatedness therefore depends on accurate pedigree records and appropriate statistical interpretation. It is a powerful method, but it represents expected genetic relationships rather than complete measurement of realized genetic sharing.
- The development of genomic technologies has not made pedigrees obsolete. Instead, pedigree and genomic information are increasingly complementary. Pedigrees provide information about multigenerational ancestry and remain essential for many breeding programs, while genomic data provide additional information about realized genetic relationships and can identify relationships that are not visible in the recorded pedigree.
- In modern precision livestock breeding, pedigree-based relatedness can be combined with phenotype, genotype, reproductive records, environmental information, and breeding values. This integrated approach can improve selection decisions while helping breeders manage inbreeding, preserve genetic diversity, and maintain long-term population health.
- The importance of pedigree-based relatedness extends beyond individual mating decisions. It provides a framework for understanding the genetic structure of an entire breeding population. By tracing ancestry and quantifying expected relationships, breeders can identify highly connected families, monitor the accumulation of relatedness, evaluate breeding contributions, and design strategies that support sustainable genetic improvement.
- Ultimately, the goal of using pedigree-based relatedness is not simply to identify which animals are related. Its greater value is to understand how common ancestry, identity by descent, kinship, inbreeding, genetic diversity, and breeding value interact within a population.
- A well-managed breeding program uses pedigree-based relatedness together with accurate phenotyping, reliable genetic evaluation, genomic information where available, and a balanced breeding objective. This allows breeders to achieve genetic improvement while controlling inbreeding and maintaining sufficient genetic diversity for future generations.
- Pedigree-based relatedness therefore remains a fundamental tool in animal breeding. It provides the foundation for relationship matrices, BLUP evaluations, inbreeding prediction, mate allocation, optimal contribution selection, conservation breeding, parentage analysis, and many other applications. When combined with genomic information, it becomes an even more powerful component of modern genetic management.
- The central principle is simple: animals with common ancestors are expected to share inherited genetic material, and the amount of expected sharing depends on their pedigree relationship and the ancestry connecting them. Understanding this principle allows breeders to make more informed decisions about selection, mating, genetic gain, inbreeding, and the long-term management of animal populations.
