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Challenges of Small Gene Pools


© 2005 Caroline Coile

No portion of this article may be reproduced without permission of the copyright holder. Reprinted with permission from Caroline Coile.

With our knowledge of genetics today, rare-breed fanciers have the advantage of foresight, and the opportunity to ensure that no single dog becomes too prevalent in the pedigrees of future generations.

I s your breed in trouble? How many founders is it based upon? What is its effective breeding size? Is it on the highway to extinction? Can it be made healthier?

Every breed is made up of a limited subsection of the entire canine gene pool. Many of these genes are associated with traits important for the function or appearance of a breed, but even more are hitchhiking along on the same chromosomes as the desirable genes. Some of these hitchhikers are deleterious genes associated with health problems. Most individuals, whether human or canine, probably carry from three to eight such "bad" genes. Humans have a huge number of recessive disorders, yet the prevalence of each
one is relatively rare because of the genetic diversity in the human population. Canine breeds tend to have a few recessive disorders, but their prevalence is often high because of the lack of diversity within each breed. The less diversity there is, the more likely individuals carrying the same recessive gene will mate, and the more likely it is that some of the progeny will inherit two copies and be affected. In addition, the smaller a population is, the greater the chance that some good genes will be lost purely by accident because of small sample size. Once lost, they cannot be recovered.

The popularity of a breed doesn't necessarily correlate with its genetic diversity. A very popular breed may trace back to just a handful of founders, and so genetically the breed may be comparatively homogeneous. Almost every breed can be considered once rare, as most have relatively, few founders. The decisions breeders made when creating these breeds are having long-term effects on what is now a huge population. Breeders of today's rare breeds are in a better position to avoid the same problems.

No matter how many individual members of a breed are registered, it is only the number of breeding individuals that count toward a breed's effective population size. The true effective population size, depends on how many bitches and how many dogs contribute to the next generation, and can be calculated as ((4) x (number males) x (number females)) divided by (number males + number females). For example, say the total population of a breed is 100, but half of them are neutered or spayed. Of the remaining 50, half are dogs and half are bitches. If each was bred, then the effective population would be 2,500/50 = 50. But more often, the bitches are bred to only the best of the dogs. If every bitch is bred to one of only five dogs, then the effective population size sinks to 500/30 = 17. (If math formulas scare you, a good general rule of thumb is that the effective population size can never be greater than four times the number of males used, and is usually somewhat less; a population with 500 bitches all bred to a single dog has an effective population size of only four.)

The more all breeders agree on what constitutes the "best" dog, and the easier it becomes to breed to that best dog using chilled semen or airline travel, the greater the chance of only a comparatively few studs being used, and the smaller the effective population size becomes. The smaller the effective population size, the less genetic diversity and the greater the chance for expression of deleterious genes will be in future generations. The more dogs that are passed by for breeding, the greater the chance that those fortunate genes that these dogs may have will be lost to the breed forever.

But what breeder in their right mind would bypass Mr. Wonder Stud to breed to Mr. Ho-Hum or even Mr. Faulty? To do so means sacrificing immediate show wins, and working toward the betterment of the breed as a whole rather than of one's own breeding stock. It also means having your fellow breeders question your sanity and taste. But somebody has to do it.

Then it gets even more difficult: It does little good to breed to Mr. HoHum if everybody with Mr. Wonder Stud's progeny, realizing they have bred themselves into a corner, rushes to breed that progeny to Mr. HoHum's progeny. Within a couple of generations, the breed becomes one big mix of Mr. Wonder Stud and Mr. Ho-Hum's genes, with nowhere to go from there.

Optimally, a breed should have distinct lines that descend from unrelated stock. Some subsection of each line would be bred to some dogs from other lines, but some core lines would remain free of the influence of at least one other line. Unfortunately, this scheme often requires more dogs than a breed's popularity can support. In breeds with large litters but few available homes, multiple-sired litters can produce greater genetic diversity within one litter, enabling the breeder to breed the equivalent of two or three litters with fewer resulting puppies.

Geneticists consider an effective population size of less than 500 to indicate that a species is in danger of extinction. By this criterion, most rare breeds would be considered in trouble. When a breed is endangered, breeders may be able to scour a breed's country of origin in search of unrelated dogs to integrate into the gene pool. In other cases, breeders have resorted to cross-breeding with a similar breed, a practice that is seldom condoned by kennel clubs.

Some breeders are hesitant to incorporate genes from unknown lines. After all, what genetic problems might be lurking there? True, there is a risk of bringing in a new set of genetic problems. However, it is better to have lower frequencies of several different harmful recessives than to have a higher frequency of one single recessive.

The lessons for rare breeds are: Maintain a standard open to interpretation, so that not all breeders have the same vision. Integrate as many dogs into initial breeding populations as possible. Try to use dogs from as many divergent lines as possible. Don't give up on a line even though it may not be a "winner." Don't abandon a dog because it has one fault, especially a recessive one for which a carrier test is available. Sometimes bettering a breed means taking a backseat to winning, in order to favor maintaining genetic diversity for the generations to come.

D. Caroline Code, Ph.D., is a breeder, owner and handler of top-winning Salukis, and the author of 29 books. Send questions to D. Caroline Coile at breedersnote

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