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Popular-Sire Syndrome Popular-Sire Syndrome: Keeping watch over health and quality issues in purebreds By Jerold S Bell, DVM, Tufts Cummings School of Veterinary Medicine (This article originally appeared in the “Healthy Dog” section of the August, 2004 AKC Gazette) An important issue in dog breeding is the determined if they were worthy of popular-sire syndrome. This occurs when continuing in future generations. a stud dog is used extensively for breeding, spreading his genes quickly The Challenges throughout the gene pool. There are two The problem with the popular-sire syndrome problems caused by the popular-sire is that the dog’s genes are spread widely and syndrome. One is that any detrimental quickly - without evaluation of the long- genes which the sire carries will term effects of his genetic contribution. By significantly increase in frequency – the time the dog’s genetic attributes can be possibly establishing evaluated through new breed-related The problem with the popular- offspring and grand- genetic disorders. sire syndrome is that the dog’s offspring, his genes Second, as there are genes are spread widely and have already been only a certain quickly - without evaluation of distributed widely, number of bitches the long-term effects of his and his effect on the bred each year, genetic contribution. gene pool may not overuse of a popular be easily changed. sire excludes the use of other quality males, thus narrowing the diversity of the In almost all instances, popular sires are gene pool. show dogs. They obviously have phenotypic qualities that are desirable, and The popular-sire syndrome is not limited as everyone sees these winning dogs, they to breeds with small populations. Some are considered desirable mates for breeding. of the most populous breeds have had What breeders and especially stud-dog problems with this syndrome. owners must consider is the effect of their Compounding this, there are several mating selection on the gene pool. At what instances where a popular sire is replaced point does the cumulative genetic with a son, and even later a grandson. contribution of a stud dog outweigh its This creates a genetic bottleneck in the positive attributes? A popular sire may only breeding population, narrowing the produce a small proportion of the total variety of genes available. number of litters registered. However, if the litters are all out of top-quality, winning Every breed has its prominent dogs in the bitches, then his influence and the loss of genetic background of the breed. But influence of other quality males may have a most of these dogs become influential significant narrowing effect on the gene based on several significant offspring that pool. spread different combinations of the dog’s genes over several generations. In some European countries, dog-breeding The desirable and undesirable legislation is being considered that limits the characteristics of the dog were passed on, lifetime number of litters a dog can sire or expressed, evaluated by breeders, and produce. If, however, certain matings produce only pet-quality dogs, but no It is important to monitor the positive and quality breeding prospects, should the negative characteristics being produced by dog be restricted from siring a litter from popular sires. While it is satisfying to own a a different line? The popular sire’s effect popular stud dog, a true measure of a on the gene pool is on the number of breeder’s dedication is how negative health offspring that are used for breeding in the information in the offspring is made next generation, and how extensively available. All dogs carry some undesirable they are being used. This cannot be traits. Based on the variety of pedigree legislated. background of bitches who are usually brought to popular sires, there is a greater At what point does a stud-dog owner chance that some undesirable traits could be determine that their dog has been bred expressed in the offspring. It is up to the enough? It can be difficult to deny stud stud-dog owner to keep in touch with bitch service when asked, but the genetic effect owners, and check on the characteristics that of a dog on the whole breed must be are being produced. considered. If everyone is breeding to a certain stud dog, the intelligent decision Some breeders will argue that the strength of may be to wait and see what is produced a breed is in its bitches, but the fact remains from these matings. If you still desire that the stud dogs potentially have the what the stud dog produces, it is possible greatest cumulative influence on the gene that you can find an offspring who has pool. There will always be popular sires, those positive attributes, and also a and that is not necessarily bad for a breed. genetic contribution from its dam that But a dog’s influence on a breed should be you may find desirable. If a popular stud gradual, and based on proven production dog deserves to make a significant and health testing. Maintaining surveillance genetic contribution to the breed, doing of health and quality issues in breeding dogs so through multiple offspring, and and their offspring, and preserving the therefore getting a mixed compliment of genetic diversity of the gene pool, should his genes, is better than focusing on a allow a sound future for purebred dogs. single offspring. (This article can be reproduced with the Wait-and-See Approach permission of the author. All breeding dogs should be health tested [email protected]) for the conditions seen in the breed. If your breed has enrolled in the AKC- Canine Health Foundation/Orthopedic Foundation for Animals CHIC program (www.caninehealthinfo.org), prospective breeding dogs and bitches should complete the recommended breed- specific health testing prior to breeding. These may include hip radiographs, CERF eye examinations, or specific genetic tests. Maneuvering the Maze of Genetic Tests: Interpretation and Utilization Jerold S Bell DVM, Clinical Associate Professor of Genetics, Tufts Cummings School of Veterinary Medicine (Presented at the 5th Tufts’ Canine and Feline Breeding & Genetics Conference) Genetic tests vary on what they are able to identify, and however no Wire Fox Terrier has even been diagnosed with therefore how they can be used in managing genetic disease. degenerative myelopathy. In the Boxer, less than 0.5% of To understand how we can use genetic tests, we have to dogs develop the disease. Testing Boxers for the DM understand the types of tests that are available, what they susceptibility gene shows 39% testing carrier, and 43% can tell us, and their limitations. testing homozygous “at risk” for the susceptibility gene. This is an example of a genetic test with low penetrance; Phenotypic tests: Some tests measure the phenotype, or indicating that the homozygous state is poorly predictive of what can be seen in the animal. This may not directly relate clinical disease. There are additional (unidentified) genes to the genotype, or the genes regulating the defect that you that must also be present to produce clinical DM. This test is are trying to manage. Screening for cataracts, ausculting for useful in ruling out a diagnosis of DM in homozygous normal heart murmurs, hip and elbow radiographs, thyroid profiles, and carrier dogs. However, selecting against 82% (“at risk” urinalysis for crystals or metabolites, skin biopsy for and carrier dogs) of the Boxer gene pool when making sebaceous adenitis, and observations on behavioral traits are breeding decisions - when the vast majority will not produce all tests of the phenotype. Most tests of the phenotype only the disorder - is detrimental to the genetic diversity of the identify affected individuals, and not carriers of disease breed. Similar situations occur in other breeds susceptible to liability genes. DM. In these breeds, breeding dogs should NOT be selected against or have their mating choices altered due to carrier or Linked-marker based tests: Some defective genes can be homozygous “at risk” status of DM unless there is knowledge linked to a genetic marker, which could be tested for. Linked- of close (first or second degree) relatives diagnosed with marker based tests do not identify the defective gene, but a clinical degenerative myelopathy. marker that lies close on the chromosome. If a crossover occurs between the marker and the defective gene during Some breeders feel that any carrier or “at risk” dogs should reproduction, the marker will no longer be linked to the only be breed to homozygous normal testing dogs. However, defective gene. False positive and false negative results will requiring that all mating be performed with dogs from only occur. Due to this phenomenon, linkage test results must be 18% of the population (following the Boxer example) would compared with results from other family members to tremendously skew the breed’s gene pool and restrict determine whether they correlate with the known genotype genetic diversity. This is unnecessary for an extremely low of relatives. Linked marker tests include those for cerebellar prevalence disease. With genetic tests for lowly penetrant ataxia in Italian Spinone and primary hyperparathyroidism in defective genes, selection should only be considered for dogs Keeshond. with families that contain clinically affected individuals. This recommendation should significantly reduce the frequency Direct mutation based tests: Direct gene tests are specific of clinical disease, as well as the frequencies of other for mutations and are a direct measurement of the contributory alleles. genotype. They can identify affected, carrier, and normal individuals. These can be run at any age, regardless of the Other examples of low prevalence but high allele frequency age of onset of the disorder. Most direct gene tests identify diseases are cord1 PRA in English Springer Spaniels (42% “at a mutation that is causative for a genetic disorder.
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