GENETIC SELECTION

USBCC Newsletter, Fall 1994

"Why should I have my breeding dogs X-rayed? I know dysplastic pups that have been born from parents that had excellent OFA ratings. If the X-ray isn't a guarantee, what good is it?" This has been a common reaction to our discussion of Hip Dysplasia in the last issue--and to the subject in general. The answer is broader than your one pup, or litter of pups, or even your own careful breeding program. The answer is in the whole breed, the population of dogs, the pool of genes that make up the Border Collie.

Every genetic trait occurs in a certain number of dogs, a certain fraction of the total breed: red dogs are a certain fraction of the breed; tricolor dogs are another fraction; black and white are another. Hip Dysplasia is present in a little over 14% . For PRA (Progressive Retinal Atrophy), the fraction used to be about 14% of all Border Collies in Britain. Each of these numbers represents a corresponding number of the relevant genes within this same breed.

The whole collection of genes within a breed is called the "gene pool." The fraction of PRA genes in the gene pool is related to the fraction of PRA dogs in the breed, but it isn't the same. PRA, like the red color gene, is recessive. This means many dogs may carry a single copy of the gene and not have the condition, and that there are many more copies of the gene in the gene pool than there are affected dogs in the breed. In this case, with 14% of dogs affected by the genetic disease (carrying two copies), nearly half the population (47%) will be carriers of a single copy, able to pass the gene on to their pups. Two of these perfectly normal dogs--each carrying a PRA gene--may produce PRA puppies.

If you breed a PRA-carrying bitch, with her perfectly normal eyes, to a perfectly normal dog, you have a 47% chance--nearly 1 in 2--that he will also carry a PRA gene, and that you will produce one or more affected pups. Statistically, one in four of that litter will be PRA-affected.

If the chances are that great with normal dogs, what good does it do to test and remove the PRA-affected dogs from the breeding population? The real result of removal of the affected dogs is not to remove, immediately, the possibility of producing PRA pups in any particular litter. The effect is on the gene pool. Every time you prevent one PRA-affected dog from breeding, you remove its two copies of the gene. If we were to remove all PRA dogs from all breeding programs, in a single generation we would change the make-up of the gene pool so much that we would produce less than 8% PRA dogs--instead of 14%--in the next generation.

The carriers would be reduced from 47% to 39%, and the chance of breeding that carrier bitch to another carrier would be 2 in 5 instead of nearly one in two. Over succeeding generations, the numbers would continue to go down. In Britain, by such a program, PRA has been reduced from 14% to only about 1%. This means that only about 18% of British Border Collies now carry single copies of the PRA gene that they can pass on to their pups.

Not only is the number of affected dogs drastically reduced, but the chances of having a PRA pup from a mating of "perfectly normal" dogs has been greatly reduced as well. This is the result of several generations of selective breeding, in which all the affected dogs have been removed from the breeding population.

The same sort of statistics would follow the enforced removal of all dysplastic dogs from breeding programs. The multiple genes which cause HD--whether they are recessive, dominant, or some of each--would gradually be removed from the gene pool. Their fraction of the total genes would be reduced, the number of dysplastic dogs would go down, and the chances of getting a dysplastic pup from a pair of "perfectly normal" parents would go down accordingly.

Genes don't spread through a breed like an epidemic disease. Changes in the gene pool can be directed by such selective breeding, but they can also be the result of something geneticists call "Drift," a sort of accidental selection. Drift happens when a single sire is used much more than any other. His sons and grandsons are used for breeding; in a few generations almost every dog in the breed carries some of his genes and he may appear many times in most pedigrees. This is how a breed is developed, using the genes of the "best" dogs to perpetuate the "best" traits. Sometimes, however, that dog also carries something else--a gene which has no particular value, like the red gene, or which is detrimental, like PRA or HD. If no one is selecting against these other traits, they can be carried along through all the pedigrees just like the herding instinct we were intentionally perpetuating.

Modern Thoroughbred racehorses are plagued with a sometimes severe condition in which they bleed from the lungs through the nose after a heavy physical exertion, such as a race. Although it has many causes, one is genetic. How did it come to be so common in a breed where it is so detrimental? One of the founding stallions of the breed, 200 years ago, was a horse named Childers. He never raced, because his bleeding condition was so serious; he was nicknamed "Bleeding Childers." His genes are in virtually every Thoroughbred horse today. Some of those genes are the ones that cause the bleeding disease. Yes, he passed on many other fine qualities we find in today's horses, but he is at the same time responsible for the continuation of a genetic disease in the breed.

So, why should you X-ray your dog before breeding? To help assure the reduction of the HD genes in the gene pool of the Border Collie, to improve the future generations of dogs, and to avoid accidentally increasing the fraction of Border Collies with bad hips through genetic drift. Good enough?

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