Dogs are second only to humans in the number of hereditary diseases identified in the population.1 Information about the prevalence and etiology of disorders in dogs may provide insight into preventative measures and possible treatments for dogs with diseases as well as for humans sharing common disorders.2 Although no single registry maintains a record of genetic disease in dogs, it has been suggested that purebred dogs are more prone to genetic disorders than are mixed-breed dogs.3 Breeding practices and selection pressures used by breeders of purebred dogs have been implicated in the perceived high frequency of genetic disorders, whereas the random mating practices of mixed-breed dogs have been suggested to increase hybrid vigor (heterosis), resulting in healthier dogs.4
The increased homozygosity expected in purebred dogs offers the potential for these animals to have traits influenced by recessive alleles in greater frequency than their crossbred counterparts. The common assumption that a mixed-breed dog is healthier would not be true if both parents carried deleterious mutations for the same disorder. Few data have been compiled to accurately assess the question of whether purebred dogs are at greater risk for genetic disorders, compared with mixed-breed dogs. In a study5 of dogs affected with hip dysplasia, no significant difference in prevalence was observed between purebred and mixed-breed dogs.
Domestic dogs are thought to be derived from 3 to 5 wolf lineages.6 Each lineage would be derived from a few common ancestors; thus, one might expect some disorders would be common to all dogs, regardless of breed. Genetic mutations that accompanied the domestication process would be expected to be widely distributed throughout the dog population, affecting dogs of any breed, including admixtures of breeds. In contrast to more distant mutations, more recent selection pressure (eg, in Europe during the Victorian era7) would influence the distribution of newer mutations, restricting those to subsets of the overall dog population. It is likely that with breed refinement for specific tasks and morphology, some mutations accompanied selection for those traits. Rigorous selection pressures to refine the breeds by inbreeding and bottlenecks4,8 would contribute to a loss of genetic diversity, thereby increasing the likelihood of recessive disorders within a breed population.
The AKC registers purebred dogs and records ancestors. Although, in 2004, there were > 140 AKC-registered breeds, 10 breeds represented more than half of the reported AKC-registered dogs, whereas the 100 least popular breeds represented < 15% of all AKC registrations.9 The less popular breeds, with many fewer dogs registered each year, would be expected to have smaller effective gene pools. For example, the current population of Portuguese Water Dogs, ranked 56th in registrations as of 2011, has been traced back to a small number of dogs, mostly from 2 kennels, with approximately 6 ancestors comprising 80% of the current gene pool.9 Breeds with smaller gene pools and reduced genetic variation are more likely to phenotypically express a recessive disorder.1
Many studies have sought to describe the prevalence of disorders among individual breeds. Often, the focus is on a single disorder and its inheritance pattern in a particular breed to define possible mutations. Yet, more global studies designed to assess the proportion of mixed-breed and purebred dogs affected with heritable disorders can prove useful toward reducing the prevalence of those disorders in the dog population. Describing disorders equivalently expressed within purebred and mixed-breed dogs may identify disorders common in the overall population and suggest approaches to reduce the prevalence. In contrast, disorders more prevalent to a particular breed may be reduced by use of concerted breeding practices.
A recent study10 found a direct correlation between disorders inherited in purebred dogs and the morphological characteristics specified in the breed standard. Although that finding underscores the fact that purebred dogs are considered at risk for disorders, it is unknown whether mixed-breed dogs have the same risk of genetic disorders that is suggested for purebred dogs. The purpose of the study reported here was to describe the prevalence of genetic disorders in the dog population as a whole.
American Kennel Club
Intervertebral disk disease
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