Hip Quality in Dogs: A Polygenic Trait in Action

Each dropping ball represents one dog’s estimated breeding value (EBV) for hip quality assessed by the PennHIP distraction index (DI) method. Lower DI values mean tighter, healthier hip joints; higher values mean looser hips that are more prone to osteoarthritis later in life. The value represented on the X-axis represents each dog’s relative genetic merit for hip quality. Dogs with more negative values have lower risk of developing hip dysplasia with age while more positive values indicate higher risk. When dogs are chosen to become parents, they each pass on to their puppies a sample half of their own genes that control expression of hip dysplasia.

Hip quality is a polygenic trait, which means it is controlled by many genes, with each one contributing a small effect to the final outcome. During puppy growth and development, each gene expresses its influence on the hip joints, with the small contribution of each gene adding to the dog’s actual hip quality phenotype. We don’t really know how many genes are involved, or how they might interact with one another, but with real pedigrees and DI datapoints on hundreds or thousands of individual dogs, we can indirectly estimate the contribution that an individual parent will pass along to its offspring. This is how EBVs are obtained.

Try switching to the % of total view above and dropping just a handful of puppies versus several hundred. At low numbers, the bars swing around unpredictably from assessment to assessment, and that instability is exactly the challenge facing a small breeding population. As a rule of thumb, the farther a program’s annual puppy production falls below roughly 100 born per year, the harder it becomes for selection to overcome the effects of chance and shift gene frequency in the direction a breeder intends.

A dog’s own observed DI score is not identical to its breeding value, with both environment and chance also playing roles. But because heritability for DI runs above 0.5 in many breeds, a dog’s DI score is still a fairly reliable window into its genetic merit for hip quality as assessed by DI. Breeders sharpen that picture further with formal breeding-value estimates like the ones illustrated here, which combine an individual’s own score with information from all its relatives.