Vulnerable Dog Breeds Blog Series - Blog 4

A coefficient of inbreeding, or COI, can be a very useful number.

It can also become a very dangerous number if we expect it to make the breeding decision for us.

When I study a pedigree, I want to know how closely related the proposed parents are and how often the same ancestors appear. COI helps me see part of that picture. But it is only part of the picture.

What does COI actually measure?

Pedigree COI estimates the probability that a puppy will inherit two copies of a gene that are identical by descent from a shared ancestor. In simple terms, it tells us something about how much related ancestry sits behind a proposed mating.

The higher the COI, the greater the amount of shared ancestry represented in that calculation. That is why COI is useful when breeders are trying to avoid unnecessarily close matings, particularly in a vulnerable breed where the gene pool may already be small.

But the number depends on the pedigree information available. A five-generation calculation is not the same as a calculation taken back to founders. Missing dogs, incomplete records and shallow pedigrees can all make a mating appear less related than it really is.

For practical guidance on the health screening that should sit alongside any COI calculation, The Kennel Club publishes a step-by-step breeder's guide, and this analysis of pedigree dog breed populations shows how these numbers play out across entire breeds rather than single matings.

A low number is useful - but it is not a green light

I naturally look for a low COI. In one of my own Clumber Spaniel planning examples, the potential mating came out at around 2.3%, which is very low. I liked seeing that number.

But I would never say, 'The COI is 2.3%, therefore this must be a good mating.'

I still want to know the dogs. Temperament comes first for me. Then I am looking at health, structure, movement, breed type, longevity, fertility and what is known about the wider family.

A low COI cannot make poor temperament acceptable. It cannot repair weak structure. It cannot tell me whether both sides of the pedigree carry the same health problem through different ancestors. And it cannot tell me whether a dog is actually a good representative of the breed.

Sometimes a higher COI can be part of a wider plan

Breeding decisions also have to be viewed across generations rather than as isolated litters.

There may be times when I would accept a higher COI because two particular dogs offer something important, both appear healthy, and I have a realistic plan to widen the pedigree again in the next generation.

That is very different from repeatedly breeding close relatives because it is convenient.

For me, COI belongs in the front pocket, not the back pocket. I want it where I can see it and use it, but I do not want it driving the car.

Pedigree COI and DNA diversity are not the same thing

This is where the subject becomes more interesting. Pedigree COI is based on recorded ancestry. Genomic diversity testing looks directly at markers in the dog's DNA.

Those tools can complement one another, but they are not interchangeable. UC Davis, for example, uses DNA markers to assess diversity and internal relatedness in a growing number of breeds.

A pedigree may show two dogs as relatively unrelated while genomic testing reveals that the breed as a whole has already lost substantial diversity. Equally, a pedigree can contain repeated ancestors while still preserving useful genetic variants that matter to the population.

The same popular dogs can hide further back

This is one reason I still like to go back through pedigrees myself.

Two dogs may look unrelated in the first few generations, then the same influential sire starts appearing again and again further back. That was one of the points we looked at in Blog 3 on the popular sire effect.

COI gives a summary number. Looking at the pedigree shows me where that number came from.

I want both.

Breed average matters, but it is not a target

Some pedigree systems show a breed-average COI. That can be useful context, especially when comparing one proposed mating with the recent population.

But 'below breed average' does not automatically mean genetically ideal. If a breed has already become very inbred, being slightly below a high average may still leave a problem.

The opposite can also be true. In a small breed, one carefully considered mating above the average may preserve a rare branch that would otherwise disappear.

Population management is about direction of travel, not winning a competition for the lowest individual number.

What I want future breeders to inherit

The reason I care about COI is not because I want a tidy percentage on a piece of paper.

I care because I want the next breeder to have choices.

If we use COI sensibly alongside pedigree depth, health information, temperament, structure, longevity and genetic-diversity tools, it can help us see when we are narrowing a breed too quickly.

But once we turn it into a pass-or-fail score, we stop thinking.

And vulnerable breeds cannot afford for us to stop thinking.

Founder perspective

I am Fiona Robertson, founder of Newflands. My perspective comes from decades of living with, breeding and studying dogs, my background as a veterinary nurse, and years spent researching pedigrees and bloodlines in New Zealand and overseas.

This series is not about pretending there is one perfect formula for breeding. It is about opening up the conversations that matter when the population is small and every generation can change the choices available to the next one.

If you have not yet read the earlier articles in this series, start with Population Genetics Made Simple: Why Genetic Diversity Matters in Vulnerable Dog Breeds and then The Popular Sire Effect: When One Successful Dog Shapes a Whole Breed.

Next in the series: When a Breed Needs New Blood.

FAQs

What is a good COI for a dog breeding?

Lower is generally preferable because it indicates less shared recorded ancestry, but there is no single percentage that makes a mating automatically good or bad. The breed population, pedigree depth, health, temperament and the long-term breeding plan all matter.

Is a COI of 0% completely unrelated?

Not necessarily. A 0% pedigree COI means no shared ancestors were found within the pedigree depth used for the calculation. The dogs may still share older ancestry, and a closed breed population may still have limited genomic diversity.

Can a low COI guarantee healthy puppies?

No. COI is one risk-management tool. It cannot rule out inherited disease, poor temperament, structural problems or unknown genetic variants.

Is pedigree COI the same as DNA genetic-diversity testing?

No. Pedigree COI is calculated from recorded ancestry. Genomic diversity tests examine DNA markers directly. Both can provide useful but different information.

Can a breeder ever justify a higher COI?

Sometimes, particularly when preserving an important rare line, but the decision should be deliberate, health-conscious and part of a longer-term plan to avoid repeatedly concentrating the same ancestry.