What COI measures
The coefficient of inbreeding, or COI, is a single figure that estimates the probability that two copies of a gene in an individual are identical by descent, meaning they trace back to the very same ancestor on both sides of the pedigree. It is calculated from the shared ancestors in a mating, so the more often the same animals appear on both the sire's and the dam's side, the higher the number climbs. A mating between full siblings, for example, produces a COI of twenty-five procent. Think of it as a measure of how doubled-up the ancestry is, not as a verdict on any single trait.
Why it matters for health
A higher COI matters because it raises the chance that a hidden recessive fault carried quietly in the line gets paired with itself and expressed in the kittens. Beyond single defects, elevated inbreeding across a population is linked to what breeders call inbreeding depression, a gradual weakening seen as smaller litters, reduced fertility, softer immune systems and lower overall vigour. None of this means a mating with some relationship is doomed, but it does mean the risk is real and cumulative. Keeping COI in view is one of the clearest ways to protect the long-term resilience of the cats you produce.
Using COI when planning
To be useful, COI must be computed over as many generations as the pedigree allows, because a calculation based on only a few generations badly understates the true figure and can look reassuringly low while deep ancestors repeat again and again. Compare the COI of candidate matings before you decide, and keep the long-term breed average in mind rather than judging your single litter in isolation. Our app includes a COI calculator in Tools that lets you test pairings quickly, so you can weigh options side by side before committing to a mating rather than after the kittens arrive.
Complementary tools
COI works best as one instrument among several. Thoughtful outcrosses bring in fresh diversity, and avoiding the overuse of popular studs matters just as much, because a single male bred to half the population narrows the whole gene pool for a generation to come. DNA tests for known recessives let you screen carriers directly and pair them safely, which a pedigree figure alone cannot do. Above all, remember that a low COI never replaces health testing. A genetically diverse kitten from untested parents is not a safe kitten, so combine diversity, screening and honest record-keeping.