What Is COI? The Coefficient of Inbreeding Explained for Dog Owners

COI (Coefficient of Inbreeding) is a number between 0 and 1 (or 0-100%) that measures how likely it is that a dog has inherited two copies of the same gene from a common ancestor. A COI of 0% means no known common ancestors. A COI of 25% means the dog is as inbred as the offspring of a parent-to-offspring mating.

Why COI Matters for Your Dog's Health

Dogs with high COI have less genetic diversity. This generally means:

Higher expression of recessive genetic diseases
Reduced immune system effectiveness
Lower fertility and litter size
Shorter average lifespan in studies across breeds

How to Calculate COI for Your Dog

Using the American Bulldog Archive

At pedigreedatabase.ca, search your dog by name. On the pedigree page, the COI is automatically calculated and displayed for 5 and 10 generations.

Search and Calculate COI

American Bulldog COI Context

The American Bulldog breed went through a severe population bottleneck in the mid-20th century, which means breed-average COI is higher than many other breeds. When evaluating a breeding or a puppy purchase, compare the dog's COI to the breed average, not to an absolute threshold. A responsible breeder actively monitors and works to maintain or reduce COI in their lines.

Frequently Asked Questions

What is a good COI for a dog?

Most geneticists recommend a 5-generation COI under 6.25% (equivalent to first-cousin parents). American Bulldog breed average COI is higher due to the breed's relatively small foundation. Individual pairings at or below breed average, trending downward, represent responsible breeding.

Does high COI mean the dog will be unhealthy?

Not automatically. COI measures probability, not certainty. A dog with a high COI may be healthy, and a dog with a low COI may have health problems. However, at a population level, higher COI correlates with reduced immune function, shorter lifespan, and higher rate of genetic disease.

Can I calculate my American Bulldog's COI?

Yes. The American Bulldog Archive at pedigreedatabase.ca has a COI calculator built in. Search your dog's name, view the pedigree, and the COI is displayed automatically for up to 10 generations.

What is line breeding vs inbreeding?

The terms describe the same thing -- mating related dogs -- at different intensities. Line breeding typically means breeding to a common ancestor 3+ generations back to concentrate desirable traits. Inbreeding refers to closer matings (half-siblings, parent-offspring). The line between them is not scientifically defined but is culturally important in breeding communities.

American Bulldog Pedigree Archive

42,000+ records. Free to search. The most complete American Bulldog lineage resource available.

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