The coefficient of inbreeding, or COI, turns “how related are these two dogs?” into a number you can actually use when planning a mating. This guide covers what the number means, where to get it, and how to use it without over-trusting it.
The most important first
- COI estimates the probability that two copies of a gene, at a random location, are identical because they came from the same ancestor.
- A higher COI means more concentration — and more shared risk of recessive conditions.
- Pedigree-based COI depends entirely on how many generations of pedigree are used — more generations, more accurate.
- Genetic (DNA-based) COI is more precise than pedigree-based COI, where available.
- Use COI to compare planned matings, not as a single pass/fail threshold.
Free interactive tool
Calculate it for your own planned litter: enter both pedigrees and see the COI and which shared ancestors drive it. Open the COI calculator →
What the number actually means
The coefficient of inbreeding is expressed as a percentage and estimates the probability that two alleles at a given gene, one from each parent, are identical because both trace back to the same shared ancestor — in other words, how much of a puppy’s genome is likely to be homozygous as a direct result of relatedness between its parents. A COI of 0% would mean no calculable shared ancestry within the pedigree depth used; a full sibling-to-sibling or parent-to-offspring mating produces a COI around 25%, and a half-sibling or grandparent–grandchild mating about 12.5%, assuming the dogs’ own ancestors were unrelated. The FCI’s international breeding strategy advises never mating siblings or parents to offspring. The Royal Kennel Club’s guidance is to aim at or below your breed’s average. There is no universally “safe” number — appropriate levels vary by breed, by the population’s existing diversity, and by how the figure was calculated — but lower is generally safer, and comparing the COI of alternative planned matings is more useful than judging any single figure in isolation.
What the kennel clubs say
Some national clubs ban these matings outright. The Swedish Kennel Club (SKK), for example, does not allow father–daughter, mother–son or full-sibling matings.
Source: Swedish Kennel Club (SKK), Basic rules (Grundregler), in force from January 1, 2026.
Last updated: November 8, 2026
Rules change, so always check with your own kennel club.
Pedigree-based versus genetic COI
The traditional way to calculate COI uses the written pedigree, counting shared ancestors back a set number of generations — but this method is only as good as the pedigree depth used: a COI calculated on five generations will understate true relatedness compared to one calculated on ten, because distant shared ancestors outside the counted window simply don’t appear in the sum. Genetic or DNA-based COI, calculated directly from a dog’s actual DNA rather than its paper pedigree, captures relatedness more completely and doesn’t depend on pedigree accuracy or depth — where it’s available for your breed, it’s generally the more informative figure.
A pedigree COI is only as good as the pedigree behind it. An imported dog with three generations on record can look unrelated to everything when it is not. Some kennel club calculators use all the records they hold rather than a fixed number of generations, so check what yours is based on. Genetic COI avoids the pedigree problem, but different labs measure it differently, and some DNA-based estimates run high. A genetic figure cannot be compared directly with a pedigree COI, or with a breed average calculated another way. Compare like with like.
In practice
Two planned matings both show a modest COI when calculated on a standard five-generation pedigree. A genetic COI test reveals one of the pairs is considerably more related than the paperwork suggested, because of shared ancestry further back than five generations. The pedigree-based number wasn’t wrong, exactly — it was simply incomplete, and the breeder is glad they checked further before committing.
(Illustrative, composite scenario.)
Using COI in practice
COI is best used comparatively: calculate it for more than one candidate mating and favor the lower figure when other factors are similar, rather than treating any single number as an automatic pass or fail. It should sit alongside, not instead of, the other tools in this area — DNA carrier testing, health screening results, and honest evaluation of both dogs — because a low COI does not by itself guarantee a healthy litter, and a moderate COI paired with two exceptionally healthy, well-tested dogs may be a reasonable, informed choice.
A large study using genetic data from over 200 breeds found that breeds with higher average inbreeding needed veterinary care more often. That is a breed-level pattern, not a prediction for one litter — which is why COI is best used to choose the less related of two otherwise sound matings.
Work on it
A few minutes here will teach you more than re-reading the article.
Ask yourself
- Do I know how to calculate or look up the COI for a planned mating?
- Am I aware of the pedigree depth behind any COI figure I’ve used?
- Do I treat COI as one input among several, rather than a single deciding number?
Find out & check
- Find out whether a genetic (DNA-based) COI test is available for your breed.
- Check the COI of your last two litters and compare them against your breed’s typical range.
Do one thing
- Calculate the COI for a mating you’re currently considering, using both pedigree-based and, if available, genetic methods.
Was this article helpful?
Anonymous. We do not store your name, email or IP address.
Sources & further reading
- The Royal Kennel Club — Inbreeding calculators. royalkennelclub.com
- The Royal Kennel Club — Manage and maintain genetic diversity. royalkennelclub.com
- Bannasch D et al. (2021), Canine Medicine and Genetics 8:12 — The effect of inbreeding, body size and morphology on health in dog breeds. doi.org
- Belanger JM et al. (2025), Genes (Basel) — Measures of Homozygosity and Relationship to Genetic Diversity in the Bearded Collie Breed. pubmed.ncbi.nlm.nih.gov
- Fédération Cynologique Internationale — International Breeding Strategies. fci.be
- Lewis TW, Abhayaratne BM, Blott SC (2015), Canine Genetics and Epidemiology 2:13 — Trends in genetic diversity for all Kennel Club registered pedigree dog breeds. doi.org
Where evidence is limited, recommendations rest on established practice and expert judgment, and may be updated as knowledge develops.
Next steps
- Related: Inbreeding, relatedness and genetic diversity · Breeding systems.
- Return to the area hub: Genetics and inherited health.