Hip dysplasia in dogs is a polygenic disease characterized by hip instability that results in secondary osteoarthritis, lameness, and physical disability.1 Medical and surgical management of the condition have economic and emotional impacts on dog owners and breeders. As a complex trait, HD is caused by genetic and environmental factors that influence expression of the primary trait and the severity of secondary osteoarthritis.2 Factors that affect expression of HD and development of secondary osteoarthritis in dogs include sex, age, and body weight.2–6 Many genes likely underlie expression of HD, most of which have a small additive effect (polygenes), but some of which likely have larger effects.4,7,8
In North America, selection methods to improve the genetic composition of dog breeds and, in so doing, improve hip joint conformation have been based on radiographic hip joint screening, semiopen (OFA) and closed (PennHIP) hip joint registries, and organized breeding programs.9–11 Through these breeding strategies, HD was determined to be heritable, and selective breeding efforts reduced the prevalence of HD.12 For example, the prevalence of HD in German Shepherd Dogs at 12 to 16 months of age decreased from 55% to 24% after 5 generations of selection,12 and the prevalence of HD in Labrador Retrievers decreased from 30% to 10%.13,14 Selective breeding is less effective when a single phenotype is used as the selection criterion15–22 than when estimated breeding values are used.23,24
Because HD has been so difficult to accurately define and eliminate in dogs, much effort has been directed at developing and comparing the accuracy of radiographic screening tests.3,25 The most widely used method for diagnosis in North America is the ventrodorsal EHR, commonly referred to as the OFA method.26 From this radiographic image, a subjective EHR score is obtained. Objective measurements include the DI,27 DLS score,3,28,29 and NA.30 Heritabilities of these traits reportedly range from 0.10 to 0.68.31
Because the objective hip joint traits and the EHR score are modestly correlated with each other at the phenotypic level,25,29,32,33 the estimates of heritabilities and breeding values derived from a multiple-trait model, which incorporates genetic and environmental correlations among the traits, would be more accurate than if they were derived from a single trait.34,35 More importantly, the genetic correlations estimated from a multiple-trait model would provide the essential values by which a selection index could be derived to integrate the breeding values of all the traits.36 Selective breeding based on these combined breeding values should be more effective in reducing the prevalence of such a complicated trait than breeding decisions made on the basis of a breeding value for a single trait.37
Although heritabilities of radiographic hip joint measurements have been investigated in various dog breeds in various environments,33,38–41 estimates of genetic correlations among values used to judge hip joint quality are limited. The purpose of the study reported here was to estimate genetic correlations among the DI, DLS score, NA, and EHR score and their heritabilities in a multiple-trait model for subsequent use in deriving a breeding value for each of these 4 traits of hip joints.
Best linear unbiased prediction
Extended–hip joint radiograph
Orthopedic Foundation for Animals
University of Pennsylvania Hip Improvement Program
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