The “Purebred Dog” Fallacy

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Yes, I am saying that there is a tremendous fallacy connected to purebred dogs. That is the fallacy of no new blood. No new dna for 50 or more generations  would not be a problem if there were enough founding fathers to keep all the  gene pool options in play and if all the original founding fathers’ dna  was still in the blood of at least some of the descendents. We know from healthy wild populations that that magic number is about 100 unrelated founding fathers. With a number that large, all the animals in the breeding pool can afford to lose some dna from their own lines, but still expect it to be in the gene pool as a whole.  All the animals can remain unrelated to any other individual except those of their own founding lines, even if some “line breeding” or over representing a certain stud was done.

The fallacy of pure breeding is to think a founding population of 10 possibly unrelated animals is enough to close the registry in perpetuity to any but the founders’ progeny. While there is a certain homogeneity that will define the breed for several generations before problems show up. It is first inevitable that in 20-30 generations every animal in the gene pool will have the same founding fathers, and all dogs will be cousins of varying closeness and second, due to the natural attrition of dna when dogs are bred, it is inevitable that in a small founding population, dna, genes, and  important  alleles, will be lost too.

Think about it. Suppose the founding population was 10 dogs, 5 females and 5 males. Each one of the breedings from those 5 pairs will discard 50% of the parental dna in the new zygote. In a litter, it is rarely the same 50% in each zygote, so most likely, one of the siblings will have the gene or allele any another pup is missing.  If all the siblings are used for breeding at least once, then the gene pool has a better chance of keeping all the genes in play longer, when there are fewer than 100 founding fathers.

One of the “rules” of purebreeding  is to delete all pups from the gene pool who are not typey enough or show an unwanted trait. This means that sooner or later, dna will be lost to the line when only one or two pups in a litter are bred. If the total gene pool has 100 or more founders, this attrition will still happen in individual lines, but it will not affect the entire breed. If a line in such a breed gets stale, there will be plenty of outcrosses available within the breed as a whole.

In cases of breeds founded with less than 100 individuals before the studbooks were closed, where inbreeding or “line breeding”  is routinely used, and all flawed pups are culled, the attrition rate of the total founders’ dna is accelerated.

The SW Vurmer Example:

Take a hypothetical breed, similar to my favorite landrace dogs,we’ll call the Southwestern Vermin Killer, aka the Vurmer, which is really based on a particular litter in the late 19th century, in which each individual was notorious for the number of vermin it killed. The pups mostly had a certain size, say 14-18 pounds,  similar coat colors and patterns,  say fawn or reddish with white bellies.  And big ears that stand up well and can be focused to hear the vermin better.  Since the people always needed better vermin killers, all the neighbors bred to the best male, “Primo”, the champion vurminater for many years in the very popular vermin killing contests of the area. And the females from Primo’s litter were all bred to other local mutts noted for vermin pouncing. So were his parents.  The best vermin catchers were bred to the best vermin catchers. The people carried on for a few generations, and noticed that if  one of the original litter esp. Primo, was in both the sire and dam’s pedigrees, then the pup had a higher chance of looking and maybe performing as well as Primo did.

A few of the Vurmer breeders decided to join the AKC and started a breed club in 1920. They registered only the preferred type of  Vurmers  and eventually  chose 50 foundation  members who best exemplified the breed (and  were mostly related to each by  the original litter’s breedings) and joined the AKC. From that point on only the registered progeny could breed with each other. The original 50 Vurmers, because they were already loosely related, actually amounted to 30 complete Vurmer genetic individuals or rather, 30 distinct ancestors, although the genes were all mixed up in all of the dogs by the time they closed the registry.

For many doggy generations, these little dogs did OK. Of course, there were always culls, some of them obviously extremely affected by the previously unknown pairing of two homozygous genes which were deeply buried in the original stock. Everyone was trying to breed to the original Primo type, so a lot of line breeding was encouraged and practiced. During the 1950’s, few breeders looked past 4 or 5 generations, but they knew that most of the dogs went back to Primo and his siblings,  and had been bred to look and be like Primo, himself. In fact, if a 30 generation pedigree could have been done in the 1960’s, some individuals would have been as high as 70% and more of Primo’s own genes. But since they were just doing 4-5 and exceptionally, 10 generation pedigrees, they could not calculate this. Also, some previously rare diseases were becoming far more common, and those dogs had to be culled  from the genepool, too.

The only other problem was that due to such strict culling, by the 1980’s there were only 15 remaining full genetic sets of genes from the original stock, although no one in the breed knew of- or even guessed- that such a thing had happened. The rest of the genes were culled and/or lost through the zygote attrition. Of course, by then, the Vurmer was extremely consistent in both looks and temperament. But.  It was becoming known that you needed a vet on staff to keep one alive past 6 years old. All kind of ailments were showing up from bad kidneys to bad heart valves and those eye-leak issues that left the stains, and nervous temperaments, too. Some people gave up on the show ring Vurmers and returned to the working Vurmers and as a result, the high of 18,000 Vurmers registered in 1994 has declined precipitously since- (although that has also been the pattern for the vast majority of other breeds, as well). The demands of the show ring were showing in that the  Vurmers of the 1990’s were smaller and more fine boned than Primo was, but the breeders and judges liked the look, even though the sporting Vurmers claimed that the show ring Vurmers wouldn’t recognize a vermin, if it bit them.

The standard was also changed to promote the more deerlike look. By 2010, the Vurmers genome is down to 6 complete individuals  among 1,500 registered Vurmers. Only people willing to pay for cesareans are still in the Vurmer ring, although the UKC is allowing the working Vurmers to show their stuff and that type is definitely more robust than the ones in the show ring.

And the Vurmers are better off than the Icelandic Collies for instance, who have a total about 3-4 complete individuals amongst their 1,500 registered individuals.

People are kicking and screaming and denying the situation now, but when the population genetics is done on their breed, they too, will start freaking out at how few founders’ genes are left in most dog populations of imported or designed breeds, such as the Chinese Crested, which no one can deny was invented from the 1930’s to the 1950’s with a very limited stock of founders.

Why is 100 founding individuals the cut off and not 50, or 30? If I could do the math, and you could understand it if I did, population genetics has a formula that calculates the attrition of genes from a closed population times the number of generations until every member has, say,  6% or more of their genome in common, which is the equivalent of 1st cousins breeding to each other. It is well known that 1st cousin breeding in people quickly results in terrible degeneration of the descendants if practiced- especially for more than one generation.

The Icelandic Collie, when it breeds, is genetically breeding to a sibling or parent or child. That is how many genes they have in common and how few other genes there are in their gene pool other than the shared ones. I predict the breed will die out, if a massive outcross program is not tried soon.

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