An outcrossed canine pedigree for linkage analysis of hip dysplasia

Canine hip dysplasia (CHD) is a prevalent, debilitating, polygenic disease characterized by hip subluxation and laxity which results in osteoarthritis . We are developing an informative pedigree for linkage analysis of CHD. Th e seven greyhound founders had excellent hip conformation with high dorsola teral subluxation scores (percentage of femoral head covered by the dorsal acetabulum in a weight-bearing position) of 66 +/- 4% (mean a SD averaged o ver both hips) and low hip distraction (laxity) indices of 0.14 +/- 0.08. N ine greyhounds bred on site had radiographic evidence of ossification in th e capital femoral chondroepiphysis at 7.7 +/- 0.9 days of age. At 8 months of age they had a mean distraction index of 0.24 +/- 0.08 and dorsolateral subluxation score of 76 +/- 1%. Of the four dysplastic Labrador retriever f ounders, three had mean age at onset of capital femoral chondroepiphyseal o ssification of 20 +/- 7 days of age (n = 3) and a mean distraction index of 0.46 +/- 0.1 accompanied by hip osteoarthritis. Thirty-four F(1)s had mean onset of capital femoral ossification (10.7 +/- 4.0 days of age) and mean dorsolateral subluxation scores (61 +/- 12%) similar to the greyhound found ers, but distraction indices (0.42 +/- 0.2) more similar to the Labrador re triever founders. One F-1 had CHD radiographically but none of 20 F(1)s had osteoarthritis at necropsy at 10 months of age. These data suggested that maximum passive laxity (as measured by the distraction index) and normal os seous conformation (as indicated by a high dorsolateral subluxation score) were both dominant traits and were controlled by separate quantitative trai t loci (QTL). Forty-three backcrosses between F(1)s with the highest hip la xity and greyhound founders had mean onset of capital femoral ossification at 9.9 +/- 2.6 days of age. Of 10 dogs in the backcross generation that hav e reached 8 months of age, 2 had palpable subluxation without marked CHD ra diographically, The mean distraction index of these dogs was 0.36 +/- 0.16 and the dorsolateral subluxation score was 65 +/- 5%, Although dogs in the backcross generation that were three-quarter greyhound had a broad range of hip laxity, a protective effect of the greyhound OTLs for good osseous con formation has mitigated thus far against subluxation and CHD.