- Genomic relatedness refers to the genetic similarity between animals estimated using DNA marker information across their genomes. In animal breeding, it helps identify how closely animals are genetically related, estimate the proportion of shared genetic material, and support decisions about mate selection, inbreeding management, and long-term genetic improvement. Unlike pedigree-based relatedness, which uses recorded ancestry, genomic relatedness estimates relationships from observed genetic variation, often using thousands of single-nucleotide polymorphisms (SNPs) distributed across the genome.
- Genomic relatedness is commonly estimated using genomic relationship matrices, which summarize the genetic relationships among animals based on their genotypes. These matrices are widely used in genomic evaluation, genomic selection, and genomic best linear unbiased prediction (GBLUP). A genomic relationship matrix can capture differences in realized genetic sharing between individuals, including variation that pedigree records alone may not reveal. However, estimates depend on marker density, allele frequencies, genotype quality, population structure, and the method used to construct the matrix.
- Pedigree-based relationships represent the expected proportion of genetic material shared because of known ancestry, whereas genomic relationships reflect the sharing observed at the genotyped markers. For example, full siblings have an expected relationship of approximately 0.5 under the conventional additive relationship definition, but their realized genomic relationships can differ because they inherit different combinations of chromosome segments from their parents. Genomic estimates can therefore help distinguish among animals with the same expected pedigree relationship. The numerical scale must be interpreted carefully because some genomic methods express relationships using an additive relationship scale, while others use alternative scaling or centering conventions.
- Genomic relatedness is closely associated with kinship, coancestry, homozygosity, and inbreeding. Animals with high genomic relatedness are more likely to share inherited alleles, and mating closely related animals can increase the probability that offspring inherit identical copies of an allele from a common ancestor. Genomic information can help breeders estimate the risk of inbreeding in planned matings and reduce the accumulation of inbreeding across generations. Nevertheless, genomic relatedness and genomic inbreeding are not identical measures: relatedness compares genetic similarity between individuals, while inbreeding concerns the probability that an individual’s two alleles at a locus are identical by descent or an estimate of that individual’s genomic homozygosity, depending on the method.
- One important application is genomic mate selection, in which breeders combine genomic relationship estimates with estimated breeding values and breeding objectives to select suitable mating pairs. This approach can help maintain genetic diversity while achieving improvement in economically important traits such as growth, fertility, milk production, disease resistance, feed efficiency, and longevity. Optimal contribution selection can also use genomic relationships to balance expected genetic gain against the risk of increased relatedness and inbreeding. This is particularly valuable in populations where a small number of high-performing breeding males may otherwise contribute disproportionately to the next generation.
- Genomic relatedness is also useful for checking pedigree accuracy, identifying unexpected relationships, studying population structure, and managing genetic resources in small or closed breeding populations. It can reveal genetic similarities that are not obvious from recorded pedigrees, including cases where pedigree information is incomplete or inaccurate. In conservation breeding, genomic relationships can help identify genetically distinct animals and guide matings that preserve variation. However, high genetic difference does not automatically mean that two animals are suitable breeding partners; health, adaptation, trait performance, genetic defects, and the overall breeding objective must also be considered.
- The reliability of genomic relatedness estimates depends on the quality and representativeness of genomic data, the number and distribution of markers, allele-frequency assumptions, and the statistical method used. Genotyping errors, missing data, population stratification, and differences in genetic background can influence estimates. Relationship values from different genomic methods may not be directly comparable unless their definitions and scaling are consistent. For practical breeding decisions, genomic estimates should therefore be interpreted alongside pedigree records, phenotypic information, estimated breeding values, and population-specific knowledge.
- Genomic relatedness is an important component of modern animal breeding and genetic management because it provides a more detailed view of genetic relationships than pedigree information alone can offer. When integrated with genomic selection, inbreeding management, and genetic diversity monitoring, it can support accurate genetic evaluation, responsible mate allocation, and sustainable genetic improvement. Its main value is not simply to identify which animals are most similar, but to help breeders make balanced decisions that improve desirable traits while maintaining sufficient genetic variation for future generations.
