Inbreeding Coefficient (COI) for Cat Breeders: What Is a Safe Number?

The coefficient of inbreeding, usually shortened to COI, is one of the most useful numbers a cat breeder can know — and one of the most misunderstood. It is a single percentage that tells you how closely related a kitten's parents are, and therefore how likely that kitten is to inherit two identical copies of the same gene from a shared ancestor.

Every pedigree breed began from a relatively small group of founding cats, so almost every pedigree cat carries some level of inbreeding. That is not automatically a problem. The issue is how much, how close to the kitten it occurs, and which ancestors are being doubled up. Used properly, COI helps you make informed choices that protect the health, fertility and long-term future of your breed.

This guide explains what COI really measures, how Wright's formula calculates it, what figures breed clubs consider sensible, and how to use COI alongside DNA testing when planning a litter.

What the coefficient of inbreeding measures

Every cat carries two copies of each gene — one inherited from its sire and one from its dam. When both parents descend from the same ancestor, there is a chance that the kitten receives the very same copy of a gene from that ancestor down both sides of the family. Geneticists call this being "identical by descent".

COI is the probability, expressed as a percentage, that any given gene in the kitten is identical by descent. A COI of 25% means that, on average, a quarter of the kitten's genes are expected to be identical copies inherited from shared ancestors. A COI of 0% (over the generations counted) means no ancestor appears on both sides.

Why identical genes matter

Most harmful genetic conditions in cats are recessive. A cat with one faulty copy and one normal copy is a healthy carrier; problems appear only when a kitten inherits two faulty copies. Inbreeding increases the chance of that happening, because carriers on both sides of the pedigree are often the same cat.

Even where no specific disease gene is involved, high levels of inbreeding are associated with "inbreeding depression" — a gradual decline in vigour that shows up as smaller litters, lower kitten survival, weaker immune systems, reduced fertility and shorter lifespans.

How Wright's formula works

The standard method is Wright's coefficient of inbreeding, devised by geneticist Sewall Wright in 1922. For every common ancestor that appears on both the sire's and the dam's side, you trace each path from the sire, up to that ancestor, and back down to the dam.

For each path you count n1 — the number of generations from the sire to the common ancestor — and n2 — the number of generations from the dam to the common ancestor. The contribution of that path is (½)^(n1+n2+1). You add up the contributions of every path through every common ancestor to get the kitten's COI.

If the common ancestor is itself inbred, its contribution is multiplied by (1 + its own COI), which is why very deep pedigrees can produce surprisingly high figures.

Worked example: half siblings

Suppose the sire and dam share the same father but have different mothers. The shared father is one generation above the sire (n1 = 1) and one generation above the dam (n2 = 1). The contribution is (½)^(1+1+1) = (½)^3 = 0.125, or 12.5%. That is the COI of a half-sibling mating before any further shared ancestors are counted.

Common reference figures

• Parent to offspring: 25%
• Full brother to sister: 25%
• Half siblings: 12.5%
• Grandparent to grandchild: 12.5%
• Uncle/aunt to niece/nephew: 12.5%
• First cousins: 6.25%
• Second cousins: about 1.56%

These are minimum figures. In real pedigrees, additional shared ancestors further back push the total higher, often considerably.

How many generations should you count?

COI depends heavily on how far back you look. A pairing can show 0% over three generations and 9% over ten. That is not a contradiction — the shorter calculation simply cannot see the older shared ancestors.

For practical breeding decisions, five generations is the accepted minimum for a meaningful figure, and many breeders work with eight to ten generations where reliable records exist. The most important rule is consistency: only compare COIs calculated over the same number of generations. A 4% five-generation COI and a 4% ten-generation COI are very different results.

What is a safe COI for cats?

There is no single figure that applies to every breed, because some breeds have much smaller gene pools than others. However, these broad guidelines are widely used:

• Under 5% over five generations: generally considered good
• 5–6.25%: acceptable for many breeds, but worth reviewing which ancestors are shared
• 6.25–12.5%: high — equivalent to cousin matings or closer; needs a clear reason
• Over 12.5%: very high — reserved for rare, carefully justified situations

Many breed clubs and geneticists suggest aiming below the average COI for your breed, so that each generation gradually improves overall diversity. If your breed's average is 15%, a mating at 8% is a real improvement, even though it would be high in a breed with an average of 3%.

Linebreeding versus inbreeding

Linebreeding is a milder form of inbreeding where breeders deliberately keep a particular outstanding cat in the background of several lines, usually several generations back. Historically it was used to fix type, head shape, coat quality or eye colour.

The difference between linebreeding and inbreeding is one of degree. Linebreeding concentrates the genes of one ancestor; it also concentrates any hidden faults that ancestor carried. Modern breeders who linebreed usually combine it with DNA testing and keep a close eye on fertility and litter health, and they introduce outcrosses when warning signs appear.

Shared ancestors matter more than the number

Two pairings can have the same COI but very different risks. A 5% COI caused by many distant shared ancestors is different from a 5% COI caused by one popular stud cat appearing repeatedly. If that stud carried a recessive disease gene, doubling up on him is far riskier.

Always look at which ancestors are shared, not just the total. Popular sires — cats used heavily because they won shows — are a common cause of reduced diversity in pedigree breeds. Pair your COI review with DNA tests recorded on each cat's health and vaccination records, so you know which shared ancestors are proven clear and which are unknown.

Practical steps to lower COI

• Calculate COI for every planned mating before committing
• Use outcross studs from different lines, including imports where practical
• Avoid repeated use of the same popular stud across all your queens
• Keep full five-generation pedigrees for every breeding cat so nothing is hidden
• DNA test breeding cats for breed-relevant conditions
• Track litter sizes, kitten survival and fertility — falling numbers can signal inbreeding depression

Reading COI alongside pedigrees

COI is only as accurate as the pedigree data behind it. Missing ancestors count as unrelated, which can make a COI look lower than it really is. Misspelt names can hide a shared ancestor entirely, because the calculation thinks two spellings are two different cats.

That is why accurate, consistent pedigree records matter. Our guide on how to read a cat pedigree explains the details you should record for every ancestor.

Checking a test mating in seconds

Doing Wright's formula by hand for a five-generation pedigree means tracing dozens of paths — slow and easy to get wrong. Purrline Pedigrees' COI calculator works out the inbreeding coefficient over 3, 4 or 5 generations for any test mating and lists every shared ancestor so you can see exactly what is driving the number.

Because it uses the pedigrees already in your account — or the ones you added in seconds with the photo-to-pedigree scanner — there is nothing to retype. Once you have chosen a pairing, plan the timing with heat cycle and mating reminders, predict likely kitten colours with the colour genetics calculator, and produce the litter's 5-generation pedigrees automatically.

Key takeaways

• COI is the probability that a kitten inherits identical gene copies from a shared ancestor
• Full sibling and parent-offspring matings start at 25%; first cousins at 6.25%
• Always compare COIs calculated over the same number of generations — five as a minimum
• Many breeders aim for under 5% over five generations, and below their breed's average
• Look at which ancestors are shared, and combine COI with DNA testing

Want to check your next pairing in seconds? Try Purrline Pedigrees free for 7 days.