Most owners know inbreeding is bad. Very few know what it actually does, which breeds carry the most of it, or how to tell whether the kitten in front of them came from an inbred line.
All three are answerable, and the third one takes about five minutes.
Pau is an animal biologist and has bred cats for over fifteen years at Bangkok Cats, a world championship cattery, where inbreeding coefficients are a hard constraint on the programme rather than a consideration.
What the coefficient actually measures
Inbreeding is the mating of related cats. That covers the obvious cases and also the ones nobody notices, such as two cats sharing a great-grandparent four generations back.
The measurement is the coefficient of inbreeding, or COI. It expresses the probability that both copies of any given gene in a cat came from the same ancestor. Zero means the parents are unrelated. Twenty-five percent is what a full sibling or parent-offspring mating produces.
Below five percent is reasonable for most breeds. Below three is better, particularly in breeds with an already narrow gene pool.
Why identical copies are the problem
Every cat carries some harmful recessive variants. In an outbred cat those stay hidden, because the second copy of the gene comes from an unrelated ancestor and works normally. One functional copy is enough.
Inbreeding removes that second opinion. When both copies trace back to the same ancestor, they are more likely to be the same variant, and a harmful recessive that was invisible for generations is suddenly expressed.
This is why COI is predictive rather than descriptive. It is not measuring damage already done. It is measuring how likely the damage is.
What inbreeding depression looks like
Weaker immune response. Immune recognition depends on the diversity of MHC genes. Less diversity means a narrower range of pathogens the cat recognises properly. Inbred cats catch more, take longer to recover, and respond less well to vaccination.
Reproductive failure. Lower fertility, smaller litters, higher neonatal mortality. Queens that struggle to conceive, carry fewer kittens to term, and lose more of the ones they deliver. Reduced sperm quality in males.
Hereditary disease showing up. The breed's specific load stops staying hidden. In Bengals that means hypertrophic cardiomyopathy, pyruvate kinase deficiency and progressive retinal atrophy. In Persians and Exotics, polycystic kidney disease. In Maine Coons, spinal muscular atrophy and HCM.
Shorter life. The compound effect of all of the above. Well-managed lines of these breeds routinely reach twelve to fifteen years. Heavily inbred lines produce cats that look old at six or seven.
Less vitality. Inbred cats are often flatter, less active and less resilient. In a show ring, the presence and energy that make a cat stand out are partly expressions of biological vigour, which is exactly what inbreeding erodes.
Why it happens on purpose
Some of it is carelessness. A lot of it is not.
Breeding close relatives fixes traits fast. Two related cats are more likely to produce offspring homozygous for the genes behind whatever the breeder is selecting for, so the kittens look more like the parents and more like each other. Done at lower intensity it is called line breeding, and it is a legitimate tool used badly more often than well.
The problem is that it fixes everything. The harmful recessives consolidate on exactly the same schedule as the desirable traits, and they do it invisibly, because the damage does not appear in the generation that caused it.
The other driver is arithmetic. Some breeds have very few registered breeding cats anywhere in the world, and in a small population unrelated mates genuinely run out. COIs drift upward even in programmes actively fighting it, which is why importing costs what it does, as set out in why pedigree cats are so expensive.
How to check before you buy
Ask for the five-generation pedigree. Look for repeated names. The same ancestor appearing several times, especially within the last three or four generations and on both the sire and dam sides, means a high COI. A pedigree where every name appears once is a genuinely outbred one.
Ask for the number. A breeder who calculates COIs answers immediately. A breeder who has never calculated one has told you how the programme is run, which is the information you wanted.
Ask for the health testing on parents and grandparents. Certificates, not descriptions. Inbreeding and untested lines are the same problem arriving from two directions, and the questions that surface both are in how to find a responsible cat breeder in Thailand.
Where this shows up in Thailand
Persians and Exotic Shorthairs from local programmes with no documented genetic management are the most common. These breeds already have limited diversity globally because of the extreme conformation they have been selected for, and a closed local population on top of that compounds it quickly.
Scottish Folds carry a second problem alongside inbreeding, and it is worth being precise about it because the terminology gets mangled. The folded ear comes from a mutation in the TRPV4 gene. That same mutation causes feline osteochondrodysplasia, sometimes abbreviated FOCD, a skeletal disease affecting cartilage and bone throughout the body. Cats carrying two copies are severely affected, with painful malformation of the limbs and tail. Cats carrying one copy still develop degenerative joint changes, often earlier and more severely than owners expect.
The fold is not a cosmetic quirk that happens to be cute. It is the visible sign of a cartilage disorder, which is why several registries decline to recognise the breed at all. Anyone considering a Scottish Fold should read about TRPV4 and osteochondrodysplasia before anything else.
Bengals from programmes with documented testing and managed COIs are a different proposition from Bengals produced locally with neither, and the hidden dangers of buying from a backyard breeder covers what the second category tends to produce.
Common questions
The pedigree shows the same ancestor twice. Is that a problem?
Not on its own. It depends how close that ancestor is and how often it appears. A great-great-grandparent twice is minor. A grandparent twice is not. The COI calculation weighs both frequency and generational distance, which is why asking for the number beats reading the chart.
Can an inbred cat live a normal healthy life?
Yes, and many do. Genetics is probability, not fate. A high COI raises the chance of problems and lowers the expected lifespan. It does not sentence an individual cat to anything. The argument is about what the practice does across a population, and what risk a buyer takes on without being told.
Is there a test for inbreeding?
COI is calculated from pedigree data rather than measured from a sample. Some laboratories offer genomic diversity panels that measure heterozygosity directly, which is a true measurement rather than an estimate. For most buyers, the pedigree and the breeder's calculated COI are enough.
Does one outcross fix an inbred line?
It lowers the COI of that litter immediately. It does not clear the accumulated genetic load, which takes several generations of deliberate outcrossing to dilute. Consistency over generations matters more than any single pairing.
How do breeders keep COIs low where the local population is small?
Importing. Bringing in cats from programmes with bloodlines not represented locally is the only reliable way to add diversity to a closed population, and each import gets evaluated for what it contributes to the COI of future litters rather than for looks alone. The cats and pedigrees behind this programme are at Bangkok Cats.
