Hip Quality: A Polygenic Trait in Action

Each dropping ball represents a puppy. Its final resting bin is that puppy’s PennHIP distraction index (DI) score, which is a measure of hip joint laxity. Tighter hips are less prone to develop osteoarthritis with age, while the opposite is true for looser hips. The inheritance of hip quality in dogs is polygenic, meaning that within each dog’s genome, there are many genes, each having a small effect, that control development of the hip joint as the puppy grows.

As each puppy grows and develops, the genes in the puppy’s genome deflect the growth and development of the hip joint either left (tighter) or right (looser) depending on the actual gene inherited at that locus from its parents. In other words, each gene contributes a small effect that nudges the developing hip slightly tighter or looser, with no single gene deciding the outcome. Because DI is a highly heritable trait in most dog breeds, the pattern of which genes a puppy happened to inherit is suggested by the DI score. For this measure of hip quality, genetics more than chance or environment is what mostly determines where the puppy’s DI score actually lands.

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.

Because heritability of DI runs above 0.5 in many breeds, a puppy’s own score is actually a fairly reliable window into its genetic merit for hip quality. Most of what causes its DI score to land in a given bin is the combination of genes it inherited, not chance or environmental factors. Breeders still get an even sharper picture by looking at the pattern across a sire or dam’s full set of progeny, not just one puppy, but an individual score here is far from meaningless.