Glossary
Inbreeding coefficient
A percentage showing how likely a dog is to carry two identical copies of a gene from a shared ancestor, a measure of how closely related its parents were.
Also called: COI, coefficient of inbreeding, inbreeding, line breeding, linebreeding, genomic COI
The inbreeding coefficient, often written COI, estimates the chance that a dog has inherited two copies of the same gene from a common ancestor on both sides of its family. It is given as a percentage. A dog from unrelated parents has a COI near zero, and a dog from siblings or a parent and its offspring has a much higher one.
Breeders can calculate it from a pedigree, where the number depends on how many generations are counted, or from a Dog DNA test that measures the shared genes directly. Higher values are linked with less genetic diversity, a greater chance of inherited disease and, in studies, smaller litters and shorter lives. Line breeding is a milder form of the same practice.
A COI is a statistic, not a verdict on one dog's health. It does not show which gene a dog has, so it works alongside health testing. Ask a dog breeder about the COI of a planned litter.
Last reviewed November 8, 2026