BREEDER MYTH- Crossbreeding will produce the disorders of both breeds in the puppies

By Carol Beuchat PhD

In any discussion about crossbreeding (mating two different breeds), somebody will surely claim that the offspring will be afflicted with the diseases of both breeds, or at the very least new mutations hiding in the gene pool will wreak havoc in future generations.

​Is this true?

Since breeding is all about genetics, let's see what science can tell us about this.

First, dominant mutations are expressed with only one copy of the mutation, so if either parent is afflicted you should be able to avoid using those dogs.

​The tricky mutations are recessive. If a dog only has one copy, it is silent. But if a dog gets two copies, then it can't have any copies of the normal gene. Whatever protein that gene was supposed to code for won't be produced, and whatever the protein was supposed to do won't happen, so that dog will have a genetic disorder.

It turns out that 70% or more of the known genetic disorders in dogs are caused by recessive mutations, so the  problem is that the particular gene is homozygous for the mutant, non-functional allele. When you breed within the closed gene pool of a purebred dog breed, all of the dogs are necessarily related, so they will share at least some alleles. More closely related dogs will have more alleles in common, and some of those will be recessive mutations that can produce a disorder. The best way to avoid producing dogs that are affected by a disorder caused by a recessive mutation is to avoid breeding dogs that are likely to have the same ones; that is, mating related dogs is risky, and the more closely related, the greater the risk. This is why inbreeding produces genetic disease, not just in dogs, but in all plants and animals.

The best way to reduce the risk of genetic disorders caused by recessive mutations is to avoid breeding related dogs, which is a problem in a closed gene pool where everybody is related. Because purebred breeds have been bred within a closed gene pool for many generations, the dogs within a breed will share many mutations, but the dogs in different breeds will not. If you are crossing Breed A with Breed B, and the breeds are not closely related (e.g, Wire and Smooth Dachshunds, English and American Cockers), then the risk of producing a disease in the puppies caused by a recessive mutation will be equal to the kinship coefficient of the pair of dogs, which tells you how genetically similar they are. (The coefficient of inbreeding of a dog is the kinship coefficient of its parents.)

So, if a breed cross will produce pups with predicted genomic COI of 2%, then the risk of producing puppies with two copies of the same recessive mutation is 2% - extremely low. As far as genetic disease goes, a breed cross has a very low risk. And because you understand the genetics of dominant and recessive alleles, you know that the risk of the introduced mutations causing a genetic disorder somewhere down the line will stay very low as long as the mutation is rare in the population, AND you avoid breeding to closely related dogs.

So, introducing new recessive mutations to a breed from crossbreeding won't be a problem unless you break both of Mother Nature's Breeding Rules:

1) Don't make a bunch of copies of a mutation and spread it throughout the gene pool. (Popular sires, we're pointing at you).

2) Don't breed to your relatives.

Get those two things right, and crossbreeding isn't a problem.

In fact, think about this - it is far riskier to breed within a closed gene pool than to crossbreed to an unrelated breed. Some breeders will claim that they won't crossbreed because "they know what's in their lines". How could they if the "bad genes" are recessive and therefore silent? So, you can reject this claim out of hand because it is inconsistent with what we know about genetics. 

If we stick to science, and ignore the arguments and excuses coming from hearts instead of heads, then crossing two breeds will not in fact produce a litter of puppies that is riddled with disease. You might not care for the conformation or other qualities of the puppies, but they will be spared the genetic disorders resulting from inbreeding that normally afflict the breed.

Next time you're at a party with your friends in the dog fancy, grab your favorite adult drink, and when there is a lull in the conversation, boldly claim that crossing two breeds is much less likely to produce genetic disorders in the puppies than inbreeding. Then just sit back, relax, and enjoy your drink.

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