Ask ten breeders to define the difference between line-breeding and inbreeding and you will get ten slightly different answers, usually with a note of defensiveness somewhere in there. It is one of the most misunderstood subjects in the cat fancy, and yet it sits right at the heart of whether a breeding programme thrives or quietly runs into trouble. As a GCCF Full Judge who has spent twenty years breeding Siamese and Orientals and sitting on breed advisory committees, I have watched both approaches play out over many generations. So let me give you the honest, plain-English version — what the terms really mean, what the numbers are telling you, and how to use them to breed better cats.
The same tool, different intensity
Here is the uncomfortable truth that ends most arguments: line-breeding and inbreeding are not two different things. They are the same thing at different intensities. Both involve mating cats that share ancestors, which concentrates genes from those shared ancestors in the offspring. The cat fancy tends to use the word inbreeding for very close pairings — parent to offspring, full siblings — and line-breeding for looser ones, such as cousins or a grandparent appearing a couple of times in the pedigree. The biology is identical; only the degree of relatedness differs. That is why a single number, the coefficient of inbreeding, is used to describe both. Once you accept that it is a spectrum rather than two opposing camps, the whole subject becomes far easier to reason about.
Why breeders line-breed at all
If concentrating genes carries risk, why do experienced breeders do it deliberately? Because it is also how you fix type. When you double up on an outstanding ancestor, you increase the chance that its desirable qualities — a beautiful head, correct ears, sound temperament — come through reliably in the offspring and breed on. Carefully judged line-breeding is one of the legitimate tools that built every established breed we recognise today. The key word is judged. Good line-breeding is purposeful: you know exactly which ancestor you are doubling up on and why, and you have weighed the gain in consistency against the cost in diversity. Mating two related cats simply because they are both nice and conveniently available is not line-breeding; it is drift, and drift is where programmes get into difficulty.
What the COI figure is really telling you
The coefficient of inbreeding, or COI, is a single percentage that estimates how much genetic duplication a kitten is likely to carry as a result of its parents sharing ancestors. A higher figure means more concentration; a lower figure means more variety. I will not rehearse the full mechanics here — we cover the basics elsewhere — but the strategic point is this: COI is not a pass-or-fail mark. It is a measure of how hard you are pressing on the line-breeding pedal. A modest figure tells you that you are nudging type along while keeping plenty of genetic room to manoeuvre. A high figure tells you that you are concentrating hard, and that you had better have a very good reason and a plan for where the diversity comes back in afterwards.
The crucial caveat is depth. A COI calculated over three generations can look reassuringly low while hiding a great deal of concentration further back. The deeper and more complete the pedigree behind the calculation, the more honest the number. A shallow pedigree flatters the figure, and a flattering figure is worse than no figure at all, because it gives you false confidence.
Diversity figures: looking beyond a single mating
COI describes one pairing. Genetic-diversity and ancestor-loss figures describe the bigger picture — how varied the ancestry actually is across the pedigree. Two cats can show similar COI figures while having very different underlying diversity, because one draws on a wide spread of founders and the other keeps returning to the same handful of popular ancestors. This is the heart of the popular sire problem: when one outstanding stud is bred to half the queens in the country, his genes saturate the breed, and a generation later everyone discovers that all the available mates are related to him. Watching ancestor diversity across your programme, not just the COI of the litter in front of you, is what separates breeders who are thinking about the breed’s future from those who are only thinking about the next litter.
- COI answers “how related are these two particular cats?”
- Diversity and ancestor-loss answer “how varied is the whole family tree behind them?”
- You want a healthy reading on both, judged against what is normal for your breed.
How to use the numbers to make better pairings
The practical workflow I recommend is simple. Before you commit a queen to a stud, model the pairing on paper first and look at the predicted COI and diversity figures together. Ask yourself three questions. Is this figure in line with, or below, what is normal for my breed? If it is higher, what specific quality am I trying to fix, and is it worth it? And whatever I do this generation, what is my plan for bringing fresh blood back in next time? A sensible programme often alternates — a touch of line-breeding to set a feature, then an outcross to restore breadth — rather than pressing in one direction until it runs out of road. Compare your figures against your breed’s typical range rather than chasing an abstract ideal number, because what is tight in one breed is loose in another.
This is precisely the work a good record system should do for you. Perfect Pedigrees calculates COI and genetic-diversity figures from deep, correct pedigrees and lets you run a trial mating for any proposed pairing before the cats ever meet — so you are making the decision with the numbers in front of you, not with hope. You can see at a glance whether a pairing leans toward concentration or breadth, and adjust before, rather than after, the litter is on the ground.
The welfare case for diversity
Ultimately this is not an academic exercise; it is a welfare one. Concentrating genes concentrates everything, including the recessive faults that every line carries hidden. Push inbreeding too hard, too fast, and you raise the odds of bringing two copies of a harmful gene together, alongside the more general decline in vigour, fertility and immune robustness that geneticists call inbreeding depression. A breed that loses its diversity loses its resilience, and that is a problem no rosette can fix. Our responsibility as breeders is to hand the next generation a breed in better shape than we found it — healthy, varied and sustainable. Using line-breeding with respect for the numbers, rather than fear or bravado, is how we do exactly that.
Plan your next pairing with the numbers in front of you. Perfect Pedigrees works out COI and diversity from deep pedigrees and lets you trial a mating before the cats meet — see it on the how it works page, then join the founding members and build for free.
Written by Ross Davies — GCCF Full Judge, breeder and exhibitor of 20+ years, and a qualified feline behaviourist. Perfect Pedigrees is a breeder’s record-keeping and pedigree tool, not a registering body.
