Genetics
If a purebred dog of breed X mated with a purebred dog of breed Y, both meeting health standards for their breed, is there a better chance the offspring would be healthier than a same breed mating because the gene pool is larger?
In terms of health alone the first answer would be that in breeding two healthy dogs it shouldn’t matter if they’re the same of different breeds, you’re apt to get healthy pups. But this doesn’t take into account the question of recessives. Suppose you breed two dogs of different breeds that both have the same incidence of a recessive health problem. The pups would have the same odds of having that health problem as purebred pups of either breed. On the other hand, suppose the two dogs were of breeds that have no recessive health problems in common. This would reduce or eliminate the odds of the puppies of having the health problems of either breed. This is the classic explanation for the theory of first generation hybrid vigor. The resulting pups should not be bred though, since they’d have a good chance of having the recessives from BOTH breeds, so the grandpups would be inclined to be worse off than the purebred offspring of their grandparents. An excellent set of articles dealing with “hybrid vigor” can be found in DogWorld, Jan 1997 by George Padgett DVM. Another very important point to keep in mind is that when a purebred carrying a genetic defect is crossed with another breed or mixed breed, the “bad” genes do NOT “go away” even though they may not be expressed in the offspring. If crossed with another dog carrying the same defect, the offspring of that breeding will demonstrate the defect.
Purebred dogs have all these diseases, though! It seems that you never hear about mixed breed dogs with problems.
Responsible breeders try to identify genetic diseases their dogs might be carrying and to eliminate them by careful breeding. It is ironic, though not surprising, that their efforts to identify and weed out genetic problems have lead some to cry “look at all the genetic diseases purebred dogs have!” A moment’s careful thought will lead you to the conclusion that mixed breeds carry the same harmful genes (their parents, or their parents’ parents, were purebreds, after all). The differences are
· with some recessive disorders (though not all genetic defects) the disease is less likely to be expressed (though it can still be inherited by offspring)
· you have lesser likelihood of ever identifying or eliminating any harmful genes your mixed breed may be carrying
Also, if you stop and think about it, many mixed breeds are simply not tested for most problems. When they get older and limp, it’s just considered old age, although it could well be hip dysplasia. When they get older and start to go blind, it could be PRA, but the owners are unlikely to test for this. It’s not that owners of mixed breeds are bad, by any means, but they are not looking for possible inheritable problems, either.
When you breed two different breeds together, what kind of variation can you expect?
Pfaffenberger’s book has some interesting data on this. He did some experiments with four different breeds. They were dogs of approximately the same size, but very different physical appearance AND behavior. The results he saw in the first and in subsequent mixed generations are pretty interesting.
Let’s look at a common crossbreeding: “cockapoos” (which are not purebred dogs, nor registered with any registry). These are crosses between Cocker Spaniels and Minature or Toy Poodles. The dogs actually vary quite a bit, some being more poodle like than others, and some being more cocker like than others. However, they are generally all a small sized, buff colored shaggy dog. If you breed two cockapoos together (not generally done), you get an even wider variation of dogs — some look like Minature Poodles, others like Cocker Spaniels. The reason for this is the recessive genes hidden in the first cross that came out in the second generation. This is actually a visual example of why “hybrid vigor” doesn’t hold.
Whitney, Leon E (DVM). How to Breed Dogs 384 pg. Many case studies on breeds, breed crosses for dominance studies, Myths and fallacies about breeding dogs, inheritance of traits (such as temperament, health, intelligence, and abilities).
