Selection Pressure in Breeding
For the most rapid genetic change, the breeder can decide to mate only the tightest-hipped dogs within the breed (those with the lowest DI) and then continue to inbreed for tight hips from there. This approach, however, will create increased inbreeding which may have adverse effects. Founding a breeding program on only a few dogs, and inbreeding on these dogs, would reduce the overall genetic diversity in the gene pool and could contribute to the loss of some desirable traits or lead to the expression of some undesirable traits. This reality affects some breeds more than others. For example less than 2% of Golden Retrievers have hip laxity in the ‘tight-hipped’ range, meaning a DI below 0.30. If one were to require that breeding candidates conform to this standard, 98% of the Golden Retrievers would be excluded from breeding, resulting in a serious reduction in genetic diversity. This breeding strategy would neither be practical nor acceptable to breeders and it is certainly not recommended by PennHIP.
To avoid the potential problems accompanying ‘extreme’ selection, PennHIP suggests a more ‘moderate’ approach which goes hand in hand with PennHIP testing. Particularly in breeds with few or no members having tight (OA-unsusceptible) hips, this moderate approach is preferable. In such breeds it is recommended that breeders choose breeding stock from the tightest half of the breed, thereby maintaining an acceptable level of genetic diversity while still applying meaningful selection pressure.
Figure: Generational boxplots showing strategy to tighten hips of offspring through judicious selection using PennHIP. By selecting from the tightest 50% of a breed as the minimum criterion, meaningful genetic change can be expected to occur without creating a genetic bottleneck. Breed X displays a range and distribution of hip laxity similar to the current status of the Golden Retriever breed. The goal of this strategy is to tighten the hips of breed X until the breed approximates the mean and distribution of hip laxity similar that of the Borzoi and Greyhound. The tighter the means of the candidate parents' hips, the greater the selection pressure, and the faster this change will occur.
(From Kapatkin AS, Mayhew PD, Smith GK: Genetic control of canine hip dysplasia. Compend Contin Educ Pract Vet 24:681, 2002.)
By breeding only dogs having hips better than the breed average (or the breed median) the overall breed average will move toward better (tighter) hips from one generation to the next (see Figure). Clearly the more selection pressure applied (i.e., the tighter the parental DI's), the more rapid the genetic change.
The PennHIP report displays a dog’s hip laxity (DI) relative to the average hip laxity for the breed making it possible for the breeder to identify dogs whose DI will apply meaningful selection pressure. By applying at least moderate selection pressure, eventually the average of the population will shift with each generation toward tighter hips. This shift will have the effect to increasingly tighten the minimum standard for breeding. By following these time-tested principles of quantitative genetics, ultimately fewer dogs will be at risk for developing OA. Understandably, more rapid genetic change could be achieved by imposing greater selection pressure or by using the technique of estimating breeding value (EBV) which incorporates the hip laxity scores of dogs in the pedigree. These strategies are recommended for the aggressive breeder wishing to achieve the most rapid hip improvement.