Wright's COI calculates the probability that, at any given gene location, the two copies a dog carries are "identical by descent" — meaning they trace back to the exact same ancestor on both the sire's side and the dam's side.
This happens when the sire and dam share one or more common ancestors. The closer those shared ancestors are (fewer generations back), and the more paths connect them, the higher the COI.
For each common ancestor found on both sides of the pedigree:
Where n₁ is generations from sire to common ancestor, n₂ is generations from dam to common ancestor, and Fa is the common ancestor's own COI. The total COI sums all contributions across every common ancestor and every path.
A sire's grandsire (gen 2 on sire side) is the same dog as the dam's grandsire (gen 2 on dam side), and is not himself inbred:
The exponent (n₁ + n₂ + 1) means recent common ancestors contribute far more than distant ones:
This is why the COI by Generation chart on the Inbreeding Analysis page is so useful — it shows where inbreeding is concentrated.
Dog Breeder Pro calculates COI to 10 generations. The PCI tells you how complete your data actually is across those generations.
Dog (or Cat) → Genealogy → Inbreeding Analysis
Wright's COI is the headline number at the top of the page, with the full breakdown below including COI Contributors and COI by Generation. Also shown on the Dog and Cat Overview screen and in exports.