The non-inheritance myth
Over the last few years, I frequently came across articles about the causes of hip dysplasia, nearly all of them written by breeders. In common, they have one assumption: there’s no hereditary component on the problem, they assure, only environmental factors. They state that peremptorily, contradicting academic research and scientific knowledge, based only upon their own experience. And that idea, with no technical grounds, goes on sedimenting and converting more people – among those, backyard breeders, who have no control, who take no responsibility for their breeding. On the other hand, it is induced and encouraged by some sectors of the pet food industry (which produce kibble that includes supplements, supposedly beneficial for the joints but having lost some of their properties during its high-temperature processing). I want to believe that the authors of such articles have the best intentions, those of breeding, and helping others breed, healthy puppies by providing them with early life conditions that minimize their risk of acquiring dysplasias. That is a total correct principle, since it is proved that factors such as feeding, weight and physical exercise do contribute to trigger or inhibit the arising of the condition. But that is not the origin.
The breeders who stand up for that concept don’t have any knowledge about genetics whatsoever. They mated healthy dogs, some of the progeny turned out with dysplasia – and, incapable of interpreting the results, they blamed external causes. Then they focused on reducing negative environmental pressure as much as possible (by providing the puppies with a high-quality regulated diet, banning slippery ground, restraining physical activity) and succeeded in breeding puppies with better hips. None of those articles mentions if subsequent matings were repetitions of previous ones or happened between dogs of the same or of different bloodlines. There’s no other data that sustains the claim. Furthermore, this theory, which stems from mere empirical observation, does not offer an explanation to a specific question: why, among littermates who are reared for several months in the same conditions, some have normal hips and others are highly dysplastic. Not everyone had that experience. I did, in my early days as a breeder, even though I always health-tested all my dogs. Realising that environmental pressure was not to blame for that occurrence, I understood that it was crucial for me to start studying genetics. Any serious breeder should do so, because without that knowledge their breeding will always be partly random and depending on luck.
The genetical origin
Hip dysplasia is a multifactor polygenic disorder (which means there is a high number of genes involved) that might be conditioned by environmental factors. As with any living being, dogs have a pair of copies of every gene, each one inherited from a parent, and those copies, called alleles, can be equal (in which case the animal is homozygous) or different (heterozygous). Among each pair of different alleles there might be a simple relation of dominance or recessiveness (in which the dominant allele sets the physical or temperamental trait it codifies for) or another type of relation, in which the recessive allele also expresses itself, limiting in a high or low degree the influence of the dominant one. In the case of genes whose relation is of simple dominance-recessiveness, the recessive allele only expresses itself if there are two of them. That’s why hip dysplasia, being caused by an undetermined high number of recessive alleles, might affect some of the offspring of two dogs with normal hips. When that happens, it means both are carriers for the same recessive alleles that codify for hip dysplasia. In that case, since every of them has, in each pair of genes that determines the hip status, a dominant allele (which codifies for normal hips) and a recessive one (dysplastic hips), the odds of every puppy turning out dysplastic is 25% (just like 25% of the litter will likely be dysplastic). Likewise, other 25% will likely inherit only dominant alleles, from both parents, and the remaining 50% will probably have both dominant and recessive alleles, just like their parents. In these two cases, the puppies will not have dysplasia – but whereas the first, being homozygous for dominant alleles, will only have those alleles to pass on to their progeny, who will all have normal hips, the second might pass on recessive alleles to some of their offspring.
As a matter of fact, the situation is much more complex than this, because a large quantity of genes/pairs of alleles is involved and the possible combinations of inherited pairs are numerous. However, the basic notion that dysplastic puppies are offspring of two carriers for dysplasia alleles is crucial to improve the breeding programme of every serious breeder, who will have that in mind for more informed and controlled future mating options. A precious help is provided by an early test, as of 16 weeks of age: PennHIP is a 3-position x-ray exam, which allows to measure the puppy’s hip distraction (that is, the maximum space between the socket and the femur’s head) and thus evaluate its odds of developing dysplasia. By having the puppies they breed early screened, the breeder might acquire precious information for their next mating options. One or two puppies (out of, say twelve) with bad hips indicate that their parents are carriers and pass on many alleles that codify for dysplasia, so the option of breeding either of them again should be carefully pondered.
On the other hand, every information about other relatives, rather than just the offspring, is equally important. If a dog with normal hips but carrier for many alleles that codify for dysplasia mates with a bitch who doesn’t have those alleles (and therefore does not pass on dysplasia), the progeny will all be normal. However, if one of that dog’s parents is dysplastic, then we’ll know he has inherited some recessive alleles that might be passed on to the progeny. Also, the existence of a dysplastic grandparent, brother or grand-offspring should cause serious pondering, due to the dog’s odds of being a carrier for some or many recessive alleles.
Table of probabilities. Each of these dogs has a dominant allele ("D", normal hips) and a recessive one ("d", dysplasia). About 50% of the litter should inherit a pair of different alleles, Dd; 25% won't have alleles for hip dysplasia, 25% will be dysplastic.
Genetic pedigree. The squares represent males, the circles females. The dogs signed as red have dysplasia. The yellow bitch is the daughter of two dogs with normal hips but her granddam on her sire's side, an oncle and a brother are dysplastic. She mated with the dog in green, whose sire has dysplasia. The yellow/green pair, both with normal hips, have eight offspring, three of which are dysplastic.