- Genomic inbreeding refers to the level of genetic similarity within an animal’s own genome, particularly the presence of chromosome regions where the two copies inherited from its parents are identical by descent. In animal breeding, genomic inbreeding is used to assess the accumulation of related genetic material, monitor genetic diversity, and reduce the risks associated with mating related animals. It complements traditional pedigree-based methods by using DNA marker information to estimate realized genomic homozygosity and, with appropriate methods, identify regions inherited from common ancestors.
- Genomic inbreeding can be estimated using several approaches, including runs of homozygosity (ROH), genomic relationship matrices, and methods based on observed and expected homozygosity. Runs of homozygosity are continuous stretches of the genome in which an animal carries matching alleles across many consecutive markers. Long ROH regions often indicate relatively recent shared ancestry, whereas shorter regions may reflect more distant ancestry, although the interpretation depends on marker density, recombination, population history, and the method used to identify these regions. Genomic estimates therefore provide information that may not be captured by pedigree records alone.
- One widely used measure is the proportion of the autosomal genome covered by runs of homozygosity. It can be expressed as:
- F_ROH = Total length of ROH / Total autosomal genome length
- For example, if the total length of identified ROH is 200 megabases and the autosomal genome length used for the calculation is 2,500 megabases, the estimated F_ROH is 0.08, or 8%. This means that approximately 8% of the genome included in the calculation lies within the identified ROH regions. The estimate depends on the ROH detection criteria, such as minimum segment length, permitted heterozygous or missing markers, and marker density, so results should be compared only when methods are sufficiently consistent.
- Another approach estimates genomic inbreeding from a genomic relationship matrix or from the difference between observed and expected homozygosity. These methods can provide useful population-level or individual-level indicators, but their numerical values may differ because they use different reference populations, allele frequencies, and statistical assumptions. Genomic inbreeding estimates are therefore not automatically interchangeable with the traditional pedigree inbreeding coefficient, which represents the probability that the two alleles at a locus are identical by descent under the pedigree model.
- Genomic inbreeding is important because increased inbreeding can raise the probability that harmful recessive alleles occur in homozygous form. This can contribute to inbreeding depression, which may reduce fertility, reproductive performance, survival, growth, disease resistance, and overall fitness. The magnitude of these effects depends on the species, breed, trait, genetic load, and management history. Some genomic regions may have stronger associations with particular traits than others, and high homozygosity does not necessarily mean that every region contains harmful variants. Genomic findings must therefore be interpreted alongside phenotype records and relevant biological evidence.
- In practical breeding programs, genomic inbreeding helps breeders monitor changes in genetic diversity across generations and identify animals that may carry substantial homozygous regions. When combined with genomic relatedness, estimated breeding values, and genomic mate selection, it can support mating decisions that avoid excessive relatedness while preserving desirable genetic merit. Breeders may use these data to choose mating pairs with lower expected offspring inbreeding, manage the contribution of popular breeding males, and prevent a small number of highly selected animals from dominating future generations.
- Genomic inbreeding is also valuable in small, closed, or endangered livestock populations, where the number of available breeding animals may be limited. Monitoring genomic inbreeding alongside effective population size, allele diversity, and the distribution of genetic contributions can help identify emerging risks and guide conservation strategies. Optimal contribution selection can be used to balance genetic gain against the rate at which relatedness and inbreeding increase. However, minimizing inbreeding alone is not sufficient: breeding programs must also consider fertility, health, production, adaptation, animal welfare, and the overall breeding objective.
- The accuracy of genomic inbreeding estimates depends on genotype quality, marker density, genome coverage, population structure, and the analytical method. SNP arrays may not capture every genomic region equally, and different breeds may require different parameters for identifying ROH or calculating homozygosity-based measures. Pedigree information remains useful for understanding ancestry and interpreting genomic results, especially when historical records are reliable. Combining pedigree and genomic information can provide a more comprehensive view of genetic relationships and inbreeding risk.
- Genomic inbreeding is an important tool for sustainable animal breeding because it helps reveal realized patterns of homozygosity and supports more informed management of genetic diversity. When used together with pedigree analysis, genomic relationship matrices, breeding values, and long-term population monitoring, it can help breeders achieve genetic improvement while limiting excessive inbreeding and preserving the genetic resources needed for future livestock populations.
