High coefficient of inbreeding and disease

One of the things I’ve read recently by a breeder of a different breed (not Cardis) is that high COIs are absolutely essential to keep disease from spreading through a population.

Here’s how that line of reasoning works.

If I have a population of dogs that does not have Spotted Snarkle disease in their pedigrees, and I know that, I should keep breeding those dogs only within their own lines because breeding outside their lines will spread Spotted Snarkle disease.

The logical outcome of this is that the entire breed becomes very segregated, with some lines “known” for Spotted Snarkle disease, others for Awl Border disease, others for Maniac Mouth, and so on.

Increasing the genetic crossover, the exchange of genes between these lines, is therefore “dangerous” because then ALL the lines have all three diseases! Oh noes!

Here’s why that’s wrong.

Let’s create two populations.

Population A has a thousand dogs. 900 of them are free from Spotted Snarkle and 100 of them have pedigrees that are “known for” it. Of the 100, fifty are carriers of the disease and 25 actually have it. Every dog who actually has Spotted Snarkle is culled. The populations are rigidly separated.

Total population year 1: 1000

Incidence of Spotted Snarkle: 25

All affected dogs are culled, but the carrier population remains segregated. As a result,

Total population year 2 (assuming identical numbers of births and deaths): 1000

Incidence of Spotted Snarkle: 25

Population B has a thousand dogs. 900 are free from Spotted Snarkle and 100 are in pedigrees that are known for it. Same situation; fifty carriers and 25 affected dogs. The populations are allowed to freely interbreed. Again, all affecteds are culled.

The 50 carriers move out into a population of 925 other dogs. Therefore, the chance of a carrier meeting a carrier moves from 50% to around 5%.

Total population year 2: 1000 dogs.

Number with Spotted Snarkle: 2.5

So has Spotted Snarkle moved out into the whole population? Yes. Is the population still better off? YES. There are many fewer affected dogs; the problem has radically improved.

That’s how maximum genetic diversity works. Even though the incidence of deleterious genes is wider spread, the chance of identical deleterious genes meeting is MUCH lower than in an inbred population.

Proponents of high COIs and rigid segregation of lines forget some very key population truths.

One problem is that they’re assuming “success” based on only a tiny, tiny, TINY fraction of the number of diseases, genetic disorders, and weaknesses that actually exist. You can’t claim success if you’ve avoided Maniac Mouth when you have no idea what other 500 genes you’ve concentrated in your little insular population. High COI is not just associated with creating or avoiding the “sexy” diseases, the big flashy ones like Addison’s disease. It’s associated with an increased risk for low-grade, old-age, late-onset diseases. Mastitis. Kidney disease. Smaller litter sizes. Lower growth rates. Senility. Worms and other parasites. Vitamin deficiencies. Lower resistance to infections. The list is VERY long.

The second, and most sad, of the things that advocates of high COI ignore is the fact that breeders lie. They lie a LOT. I am thankful to have mentors in both Danes and Cardis who have been honest with me, but by the end of my time in Danes I could have written a book about breeders lying about pedigrees and about health. I hope things are different in Cardigans, but when when I was breeding my last Dane litter and was looking at pedigrees of stud dogs between a third and a half of them I was rejecting because I knew that at least one dog, or one health result, was false or at least very suspect. The tragedy of this, beyond the moral downfall of that breeder and the sadness and pain she causes to everyone who breeds based on a lie, is that you really can’t count on your “safe” pedigree. Not to any great extent.

If the goal of an entire breed is to keep COI low, the overall incidence of disease will be lower even if you were deceived in a pedigree or by a breeder (or that breeder’s breeder, ad nauseam). You will keep disease expression lower across the board, protecting future generations from the hundreds of diseases we can’t test for and that don’t make the headlines. And you will keep the population’s ability to respond to new, unknown threats at its highest level.

Another, more sentimental, benefit from my point of view is that spreading genes around keeps you concerned about the whole breed, and links you to and bonds you with many more breeders. It keeps the world small. You won’t be as willing to cut people down if you know they’re part of your pedigrees and you’ll be concerned with the eradication of disorders across the whole breed because your dogs participate across the whole breed.


One thought on “High coefficient of inbreeding and disease

  1. What happens though, when you end up with a disease gene that simply ends up super-widespread in a relatively diverse population? A low COI won’t help you then. Collies have a decent gene pool compared to many other breeds (The American fancy was well-established before the world wars). Eliminating affected dogs WOULD produce a bottleneck, and even simply breeding for non-affected carriers is going to bottleneck things to some degree.)

    I don’t know the answers to these questions and I don’t know who to ask to find out. (Also, any books you can recommend on population genetics? I’m curious.)

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