Gene pools, popular sires and closed studbooks
4 min read · Updated
A great many of the breed profiles on this site end up saying some version of the gene pool is small, ask what the breeder does about it. It is the least visible thing wrong with pedigree animals and the hardest to fix, because unlike a single testable disease it is a property of the whole population rather than of any one animal in it.
What a closed studbook means
Almost every pedigree breed is registered in a closed studbook: to be registered, an animal must have two registered parents. No new blood enters, ever.
That has been the definition of a breed for roughly a century and a half, and it is a recent invention rather than an ancient one. Before it, working animals were bred to whatever produced good working animals.
The consequence is arithmetic. A closed population loses genetic variation every generation and cannot regain any. Every mating is between relatives — the only question is how distant — and the average relatedness rises steadily with time.
Some breeds started from very few animals. The Norwegian Lundehund descends from about six dogs. The Birman was rebuilt after the Second World War from a handful. The German Pinscher was reconstructed from four oversized Miniature Pinschers and one smuggled bitch. Those breeds began the closed period with almost nothing to lose and lost some of it anyway.
The popular sire effect
This is the mechanism that does most of the damage, and it is entirely a human decision rather than a biological accident.
A male wins something, or throws a striking litter, and everyone breeds to him. In a few years he has hundreds or thousands of descendants, and in horses, with frozen semen, one stallion can have tens of thousands.
Two things follow. His genes crowd out everyone else's, so the effective size of the gene pool collapses far below the number of animals actually alive. And any recessive disease he happens to carry is distributed across the entire breed at once.
That is not hypothetical. HYPP in Quarter Horses traces to one enormously influential halter stallion. A number of dog breeds can point to a single dog behind a condition that is now everywhere in the population.
The useful figure here is effective population size, which counts breeding animals weighted by how much each contributes. A breed with fifty thousand registered dogs can have an effective population in the low dozens.
Why it matters to a buyer
Inbreeding raises the chance that an animal inherits the same recessive variant from both parents, which is what turns a harmless carrier state into a disease.
It also produces inbreeding depression, which is more diffuse and harder to point at: smaller litters, higher puppy and kitten mortality, poorer fertility, weaker immune function and shorter life. None of those shows up as a named condition with a test, which is exactly why they get less attention than the diseases that do.
And DNA testing, useful as it is, does not solve this. A test finds the variants somebody has already identified. It says nothing about the ones nobody has found yet, and a population with little variation left will keep producing new ones.
What the sensible breeders do
They calculate a coefficient of inbreeding for a proposed pairing and prefer matings below the breed average rather than above it. Most kennel clubs publish a calculator that will do it from the pedigree database.
They avoid the popular sire, deliberately, and use less fashionable males who are perfectly good.
They use the whole population rather than one country's fashionable lines — importing, or using semen from elsewhere, keeps distinct lines in play.
They breed from carriers when the alternative would be discarding a quarter of the gene pool, pairing them only with tested clear animals. Excluding every carrier of every testable condition is how a small breed gets smaller, and it is a mistake that well-meaning buyers encourage by asking only for clear-to-clear.
And where a breed is genuinely in trouble, some registries now permit controlled outcrossing to another breed. The Dalmatian backcross project, which introduced a Pointer to remove the urate stone problem, is the best-known example and was fiercely resisted before it was accepted.
What to ask
Ask for the coefficient of inbreeding for the litter, and how it compares with the breed average. A breeder who has never calculated it is telling you something.
Ask what they do to keep diversity in their lines. A good answer names specific dogs, imports or outcrosses. A vague one about healthy lines is not an answer.
Ask how many litters the sire has produced, and how widely he has been used.
Ask about longevity and cause of death in the grandparents, which is the cheapest proxy for the diffuse problems no test detects.
And consider the crossbreeds and the landraces honestly. A cat from a natural landrace such as the Aegean or the Kurilian Bobtail, or a well-bred crossbreed, starts from a broader base than a rare pedigree, and that is a genuine health advantage rather than a consolation.