A number of people have asked about the genetics of Hip Dysplasia. Should I refuse to breed my male dog to any bitch with less than Good or Excellent hips? How many dysplastic pups should I expect if one parent has bad hips? Should I breed a dog with severely dysplastic siblings? Does HD result from inbreeding?
Unfortunately, the answer to all of these questions is "I don't know." As in most of genetics, breeding good hips is largely a matter of chance, or probability. Unlike more simple genetic traits, however, good (or bad) hips don't result from a single pair of genes. With a single pair, the probability is easy to measure. For instance, we know exactly how many red pups, statistically, to expect from any combination.
Hips, however, are affected by a large number of genes: some may be recessive like the b that causes red color; some may be dominant like the B that causes black. The problem is that we don't know how to identify any of them, and we have no idea how many there are. If we had hip measurements on all the members of hundreds of litters and their parents and offspring, we could make a start. We aren't even close.
The genetic situation goes something like this: there is some indication that HD might be caused by a combination of three different recessive pairs, call them aa, bb, and cc. Remember that every dog carries two copies of each gene and passes only one on to each pup. A dog that has the combination AaBbCc may pass on ABC, ABc, Abc AbC, aBC, aBc, or abc to any one pup. To form a single pair, again as in the case of the red dogs, the deleterious gene must be inherited from both parents. This would mean that mother and father would each carry at least one recessive copy of each of the three genes, and pass one of each to a particular puppy--that pup would then be dysplastic. Other pups in the same litter might receive only one of each, or one a and one b and no c at all, or any of many, many combinations. The abc pup could pass all three to the next generation and produce dysplastic pups in turn if he is bred to another dog carrying all three recessives. The ab pup could never produce dysplastic pups because he doesn't have a copy of c. (He can only get one copy from each parent.) All these genetic combinations, and others, could occur in the same litter.
It is also possible, though, that the genes which in combination produce hip dysplasia are dominant--A, B, and C. In this case, only one copy of each is necessary, and each one would only have to come from one parent. A dysplastic dog, AaBbCc, could easily pass all three to one or more puppies in a litter, with no genetic help from the other parent.
We all know that two perfectly "normal" dogs can produce dysplastic pups. How is this possible? If the genes are recessive, this would be the common genetic condition, exactly as two black dogs can produce a red puppy. If they are dominant, one parent may carry, say, A and B; the other may carry only C. Neither parent has the combination that creates dysplasia. Together, they may pass on to one or more puppies the three dominant genes that make bad hips.
The probability of creating recessive combinations (aabbcc) is greater if there is a lot of inbreeding. Genes follow family lines and the same ones are simply more likely to be found in other members of the same lines.
The chance of combining several independent genes (ABC), however, is greater in cases of outbreeding. No matter how many times you introduce into a pedigree a family line carrying A and B, dysplasia will not occur unless you outcross to a family carrying C. Since we have no way of recognizing any of these until they come together, we can't know what lines to avoid crossing--until it happens. Certainly, if a particular pair of dogs produces a dysplastic puppy, we know the possibility is there; the combination shouldn't be repeated. Unfortunately, of course, we don't know until the puppy has grown up.
In either case--and I stress that both are hypothetical and that the truth is probably more complicated than either--the severely dysplastic dog should not be bred. I hear breeders say that a really good working dog should be used anyway, and the problem sorted out in later generations. It is exactly the really good dog with really bad hips that is the greatest danger. If he produces a lot of top quality trial dogs and enters the pedigrees and the major bloodlines of the breed, his dysplastic genes will be spread throughout the breed. Sooner or later, his descendants will begin to be bred back to each other.
If the genes are recessive, we know what multiple linebreeding can do--that's why Wiston Cap has given us an increase in red dogs. If they are dominant, you have created a line in which all three dominant genes are rattling around. From time to time a descendant of our AaBbCc dysplastic dog carrying only A and C will be bred to a descendant carrying B or maybe A and B. This pair is going to produce some pups with bad hips. The more crosses there are in your pedigree back to this ABC dog, the higher the probability that the three genes will come together.
Actually, for all the excuses we use for the high incidence of dysplasia (it's the pet/obedience owners; we're feeding them too much, etc) the usual cause for a high frequency of a detrimental gene in a breed is that it occurred in an extremely popular sire whose genes are now everywhere in the breed.