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Maintaining and Improving Breeds BELL, cont’d s Genetic studies of dog breeds show that they lose sire or multi-generational sire line, other quality l o on average 35% of their genetic diversity through male lines are abandoned. This causes a loss of ge - o breed formation. Genetic studies also document the netic diversity to the breed gene pool in exchange P increased homozygosity found in dog breeds. Low for a rapidly increasing influence of the popular sire. e n effective population size (low number of Now is an important time to use frozen semen of e founders) and high deep-pedigree inbreeding quality dogs from the past to expand gene pools. G coefficients (homozygosity) are a natural and Stored DNA (such as from the OFA CHIC reposi - expected consequence of breed development. tory) or semen can be used for breed-specific ge - Breeds differ from natural populations in that only netic testing that might not have been previously a small percentage of dogs reproduce to create the available. next generation. In a population sense, this repre - All individuals carry some deleterious genes, sents a genetic bottleneck with each generation. In - which can increase in frequency with natural as well dividuals chosen for breeding should represent the as artificial selection. More “lines” of naturally oc - quality traits of the breed. curring species have died Quality traits should not be off due to genetic disor - lost through the absence of The popular sire syndrome is the ders or diminished fitness selection or the abandon - single most influential factor in than those that have sur - ment of quality lines. restricting breed gene pool diversity vived. As individuals prop - Population expansion is agate, deleterious an important aspect of mutations can become breed maintenance. If the offspring of small popula - breed-related disease if they are disseminated and tion breeds are generally healthy their population increase in frequency. can grow and expand. They are at stages of breed Studies show that some breeds have more issues development where more populous breeds were of specific genetic diseases with linebreeding and earlier in their development. Breeders of small pop - others do not. This depends on the genetic load of ulation breeds need to mentor their puppy buyers to deleterious recessive genes in the gene pool. The expand their breeder base as well as the number of genetic health of dog breeds is not a direct dogs. function of homozygosity, genetic diversity, or Population expansion allows the creation of new population size; but of the accumulation and “family lines.” A larger population allows average re - propagation of specific disease liability genes. latedness of breeding pairs (based on recent gener - Artificial selection to maintain breeds requires ac - ations) to be less than the prior generation. tive selection against deleterious genes. This is eas - Population contraction is detrimental to breed main - ier with dominant or additive genes, as the genotype tenance due to the loss of quality breeding lines and is observed in the dog’s phenotype. For recessive genetic diversity. Healthy breed gene pools re - deleterious genes, selection involves the develop - quire expanding, or large, stable populations. ment and use of genetic tests that reveal the carrier There are times when a lot of breeding is going state, or the identification of lines with carrier risk. on and registrations are increasing, and times (such Some hereditary disorders and disease-predispos - as the recent past) when less breeding is going on. ing phenotypes have been actively selected for by However, it is the offspring that reproduce (regard - breeders. The most evident and widespread is the less if from prolific or limited-breeding parents) that brachycephalic obstructive airway disorder, seen in contribute their genes to the next generation. extremely short-muzzled breeds. Other extreme Breeding quality dogs from different “lines” and phenotypes include excessive skin, excessive skin areas of the gene pool prevents the loss of genetic folds, excessive hind limb angulation, excessive diversity. size, excessive coat, dome-shaped skulls, and eyelid The popular sire syndrome is the single most in - abnormalities. It is important that breed stan - fluential factor in restricting breed gene pool diver - dards and selection practices specifically avoid sity. When a breed is concentrating on a specific selection for extreme phenotypes that cause PERSPECTIVES • 9 • September 2016 BELL, cont’d s disease liability. For the show ring, judges’ ed - for breeding with a quality, healthy offspring. l o ucation should be directed towards rewarding o moderation of disease-related extreme pheno - In small population breeds with high frequencies P types. of genetic disorders, breeders are often “frozen” e n Regular breed health surveys should be con - from breeding for fear of producing disease. This e ducted by breed clubs to monitor for the presence causes continued breed decline due to population G and changing prevalence of genetic disorders. The contraction. Breed improvement requires selection OFA offers on-line health surveys for breeds. of the best breeding choices in matings that can re - Breed genetic health should be judged on duce the frequency of genetic disease. As the popu - breed health surveys that document the occur - lation and breeding choices expand, the ability to rence of genetic disease. reduce the frequency of disease expands with it. Parent breed clubs should determine realistic pre- Breed improvement involves; 1) selection of breeding genetic screening requirements based on breeding dogs, 2) appropriate pairing of the prevalence and severity of testable disorders in mates, 3) breeding, and 4) replacement of less the breed. Health testing requirements should desirable breeding dogs with more desirable be listed in the OFA CHIC and AKC Bred with offspring. H.E.A.R.T. program websites. An unfortunate development in dog breeding is Without direct selection against genetic disorders, recommendations designed for the preservation of the genetic health of breeds will decline. Breeders rare and endangered species. These involve out - who refuse to do pre-breeding health screening breeding (reducing homozygosity and average in - should be directed to find a different hobby or pro - breeding coefficients) and increasing minor gene fession that they can actually be good at. It is not or chromosome segment frequencies. Dog breed - ethical to breed dogs without selection for genetic ing requires diverse lines, and not a homogenized health. Selection of healthy breeding stock is and randomized outbred population. Outbreeding the most important aspect of maintaining will not reduce the frequency of breed-related ge - breeds. netic disease, as the causative genes are already Each breeder must prioritize their selection for dispersed in the breed gene pool. Genetic selec - positive traits and against disease traits with each tion for quality and against undesirable traits is mating. Some breeders feel that genetic screening what causes homozygosity and reduces the fre - will reduce the genetic diversity of breeds. The quency of minor genes and chromosomal seg - proper use of genetic screening actually in - ments. Blindly selecting for them without knowing creases breeding choices by allowing quality their effect could significantly reverse selection- dogs at higher-risk of carrying disease liability based breed improvement. Homozygosity is syn - genes to be bred: onymous with pure breeds. It is not inherently correlated to impaired genetic • Quality carriers of testable disease-causing reces - health, and does not have to be artificially sive genes should be bred to normal testing controlled. mates and replaced for breeding with quality, nor - Expanding populations with different breed - mal testing offspring. ers undertaking different types of matings and • Quality dogs with a less desirable phenotype selecting on different lines, while monitoring (such as fair or even mild hip dysplasia in breeds and selecting against genetic disease provides with high frequencies of dysplasia) should be for a healthy, diverse breed gene pool. bred to dogs with desirable phenotypes (good or Official genetic screening results should be made excellent hips) and replaced for breeding with off - available to prospective breeders, and to the pet and spring whose phenotype is better than the parent. breeding-stock purchasing public. This is facilitated • Quality non-affected dogs from lines expressing disorders that do not have genetic tests (such as through open genetic health databases like the epilepsy) should be bred to mates from families OFA. It doesn’t matter whether a breeder is a large or litters not expressing the disorder and replaced commercial breeder, or only breeds once. It is no PERSPECTIVES • 10 • September 2016 BELL, cont’d s longer acceptable to say that genetic disease “just as universal as equine pre-purchase examina - l o happens.” In today’s environment, not testing for tions. o documented breed-related hereditary diseases is ir - This article can be reproduced with the permis - P responsible and unethical breeding. Breed-specific sion of the author; [email protected] e n pre-breeding health screening should become e G BREED MAINTENANCE AND IMPROVEMENT REQUIRES: • A large or expanding breed population • Avoidance of the popular sire syndrome • Avoidance of extreme phenotypes that can produce disease liability • Monitoring of health issues in the breed • Constant selection
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