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In the last 200-300 years, intense artificial selection has led to the development of great number of dog breeds. Today this number exceeds 400 diverse breeds. Although phenotypic variabilities between breeds are large, breed gene pools are reduced because of intense selection and passing through bottlenecks. Instead of a greater number of genes that have minor effects, in today’s breeds a small number of genes each with a major effect on various inherited traits is present. A reduced gene pool, the tendency to use inbreeding for the purpose of breed standard maintenance and the frequent use of popular sires have all led to the emergence of great number of inherited diseases.
In 1868 Darwin discussed the diversity of physical and physiological traits in dogs of different sizes. But not until recently has significant attention been paid to inherited diseases in dogs. Inherited diseases in dogs include metabolic defects, immune system abnormalities, congenital physical deformities, neurological and sensory disorders and blood disorders. Most inherited diseases in dogs are monogenic and have an autosomal recessive mode of inheritance. Autosomal dominant and X-linked modes of inheritance are less common.
According to some researchers, the greatest number of inherited diseases is found in German shepherds, Golden retrievers, German boxers, Labrador retrievers and English springer spaniels. Breeds least affected with inherited diseases are Rhodesian ridgebacks, Staffordshire bull terriers, Whippets, Flat coated retrievers and Dogue de Bordeaux. It has been noticed that immune and musculoskeletal disorders are more associated with taller and heavier breeds, while neurological disorders are associated with the shape of the skull. Although currently, inherited disorders in purebred dogs are being researched intensively, there are still many with an undetermined mode of inheritance, occurrence and specific genetic abnormality.
The emerging development of methods in molecular biology and the canine genome sequencing project have led to recognition of a large number of mutations causing inherited diseases in dogs, and of those affecting phenotypic traits, such as coat colour or hair length.
Recognition of disease-causing mutations has led to the development of DNA tests for dogs that can give disclose the dog’s level of predisposition to the development of an inherited disease. As the number of newly identified mutations increases, so does the number of genetic tests available for dogs.
Genetic testing in dogs enables diagnosis of a disease before the development of its first symptoms. Results of genetic tests are accurate and more reliable than conventional methods for disease detection, especially in case of late onset diseases. DNA tests also enable diagnosis of the disease before reaching mating age, thus allowing prevention of the disease-causing mutation from being transmitted to offspring.
Genetic testing in dogs is the only way of revealing carriers for a disease with an autosomal recessive mode of inheritance. This is the factor that makes DNA tests a valuable weapon in the fight against inherited diseases in dogs, especially since autosomal recessive diseases are the most common.
Dogs can be genetically tested at any age and there is no need for repeated testing which makes genetic tests economically valuable. It should be noted that the test is non-invasive; a buccal swab is sufficient for performing a DNA test.
The overall goal of breeders is to breed healthy, long-lived, companionable pedigreed dogs that retain the features of traditional merit currently valued in the show ring.
Results of genetic tests are a useful tool for eliminating inherited disorders from the breeding population which makes the results a significant contributory factor in achieving the goal of breeding healthy dogs. Knowledge of the mode of inheritance of the disease-causing mutation along with the genetic test results can help the breeders to decide about future breeding pairs and to plan a breeding program. The population size and the occurrence of a disease and disease-causing mutation in any given breed should also be taken into account. If a population is small and/or a mutation is frequent in a breed, in the case of autosomal recessive diseases, breeders should be counselled, encouraging them to include carriers in the breeding population for at least a generation, in order to avoid reducing diversity unnecessarily.
As stated earlier, genetic testing in dogs allows for the early recognition of a predisposition towards the development of an inherited disease, thus affording more possibilities in terms of prevention and treatment. With knowledge of the presence of a disease-causing mutation, dogs under regular veterinary surveillance can be secured longer and healthier lives and the onset of disease can be delayed with appropriate and timely treatment.
Genetic testing in dogs is being integrated in screening programs for inherited diseases, with a growing number of countries requiring the results of DNA tests as part of the registration process of kennel clubs and show presentations.
Controlling polygenic and multifactorial conditions with a complex etiology is still difficult, despite the efforts of researchers and breeding strategies based on phenotype.
Mellersh, C. (2012). DNA testing and domestic dogs. Mamm. Genome 23, 109–123.