Hybrid Vigor - true of false

Yes and no.

Hybrid vigor basically (in terms of domestic animals) means crossing two individuals with widely divergent genepools in order that potentially harmful recessives are less likely to show up, and that animals will display more 'vigor'- faster growth and weight gain, increased overall size, and theoretically, better resistance to disease. (see http://cynography.blogspot.com/2009/05/three-hybrids.html - for some examples.)

The thing is in dogs, most of our dogs' genepools aren't actually all that divergent. All the water retrievers - poodles and labs and goldens among them- are relatively young gundog breeds, mostly bred in the UK and Germany. Most of the toy breeds that are commonly crossbred (toy poodles and basically anything else) were crossbred among themeslves if you go back a century or two. And if you're crossing two unhealth tested dogs, whose gene pools both carry all the same bad genes, such as luxating patellas, HD, overbites, liver shunts, addisons- you get the idea- are they really going to have ANY better chance of being healthy than two carefully health tested purebred dogs? Not really.

Now, there IS an exception. If you're talking strictly survival, there is something to be said for hybrid vigor. Multi-generational randombred mutts that are selected by nature (ie, are feral) will all pretty much defalut to a 40ish pound, yellow, prick eared dog. That shape and size is obviously useful from an evolutionary point of view. And really serious defects that prevent puppies from surviving (cleft palatte comes to mind) will eliminate those genes from the gene pool. But stuff like HD, LP, PRA- probably won't, because those don't affect an individual animal's ability to survive until later in life- after he or she has reproduced (which, from nature's point of view, is all the body needs to last for.)

Dogs AREN"T natural. They're a product of selective breeding, with various pressures for varoius qualities, over the course of thousands of years. Closed registry systems are a century old* and may (or may not) be a good thing in the long run. So the answer isn't really yes or no, but 'sort of' and 'maybe'.

I will tell you, though, that the two least healthy dogs I've owned have been mutts.

*Will closed registry systems survive? I don't really think so. But right now, I *do* think that responsible breeders in the vast majority ARE the people who are using those registries, and I would recommend someone go to a very serious reputable breeder of a closed-registry breed LONG before I would try and seek out someone attempting to breed a hybrid for health. I'd like to see us go to a system where each breed had allowed outcrosses, you had to apply for a litter permit with the registry to do an outcross litter, all helath testing would have to be done, and then the offspring would have a letter appended to the end of their registration number for 5 breeds. The 6th generation would be considered purebred again. The 1-5th generations from that outcross could only be bred to dogs that were NOT within 5 generations of an outcross breeding, so breeds would remain relatively pure and predictable but still be able to benefit from outcrossing.

It has always baffled me that it's not more like horse breeding.

To breed my mare and have her foal be acceptable I first had to get her inspected. She passed her inspection (actually came in reserve champion at her inspection site *minor brag*) that is done by two judges from Germany who spend the summer traveling the country inspecting horses.

Following the inspection we had two choices. Either she has to pass a performance test to get her final permanent breeding license or she has to have a foal by the time she is five that also passes inspection (to show that she breeds true). We are actually going the performance route with her and she has her final inspection this summer.

She can only be bred to stallions that were inspected and they have even stricter inspections to go through since they can have a much bigger impact (can breed many more times).

There is also a way around inspections if your horse has completed certain performance qualifications.

The breed also allows in a certain amount of outside blood that has passed inspection and proven that they would be an asset to the breed. The first generation crosses are generally not that highly desireable, but the third and fourth generations are.

Hybrid vigor does not exist within dogs. All breeds belong to the same species, so mutts are not hybrids. You'd think a biologist would know that.

Also, if my mom came from Asia and my father from Europe, and they both had cancer in their families, then I'd also have a high chance of getting cancer some day, regardless of how "unrelated" my parents are. People fail to realize that all breeds can be affected by the same genetic diseases, there are no immune breeds. And it's unlikely that breeding mutts will result in "the best of both worlds." So breeding a collie with eye problems to a lab with hip displaysia won't result in perfectly healthy pups, there will probably be a mix of BOTH problems distributed amongst the pups.

Also, it kind of seems like some people think all purebred dogs are inbred. It's not like every member of a certain breed is related to one another, that's like saying all British people are related. Though I'm sure there are SOME extremely rare, underpopulated breeds that have to use inbreeding to keep the breed alive, but the vast majority of breeds has PLENTY of genetic diversity.

To the author: No apologies necessary!! I love this subject and am more than happy to discuss.

Nargle: FYI: With very few exceptions, species cannot interbreed. I am well aware that dogs are all of the same species, whether great dane or chi. However, the word "hybrid" doesn't necessarily imply interbreeding between two species; see dictionary definition: http://dictionary.reference.com/browse/hybrid. "Hybrid vigor" in corn was not produced by crossing it with some other plant, but simply by crossing it with different varieties of the same plant. You can produce "hybrid vigor" in a dog (although it's not generally called this among breeders) by crossing different breeds. Please don't imply that I don't know what I'm talking about when it comes to this subject; I've taught several university-level courses which covered genetics.

Ok, now on to the real question...

I will demonstrate what I meant in the original quote with an example:

I own a border collie mix. I don't know what she's mixed with, but she's clearly not purebred. I know nothing about her lineage and would sincerely doubt if any genetic testing was done on her parents. Despite this, there are certain heritable disorders that she won't get. How can I possibly know this?

If I owned a purebred border collie and I knew nothing about its parents, I'd probably be worried about collie eye anomaly (CEA), a somewhat common problem in collies that results from poor breeding practices. However, this disease is caused by having TWO copies of a recessive allele for the gene, one from each parent. Since only one of my dog's parents was a collie, she can't get it, even if that parent had the disease! See http://www.bordercollieclub.com/bordercollie_glaucoma.aspx for a reference on this, and for more info on CEA.

To play devil's advocate, I suppose that it's possible that each of my dog's parents was part border collie AND that each carried the gene (i.e. had one copy of the allele). In this case, her chance of getting the disorder is 1/4.

I fully recognize that lots of diseases that dogs suffer from have different patterns of inheritance than CEA (i.e. double recessive). Having parents of different breeds doesn't protect your dog from any of those diseases. Am I worried about my mutt getting hip displasia? You bet!

I also recognize that lots of breeds suffer from similar diseases, so if your mutt is golden X lab, you won't be seeing much hybrid vigor. But CEA is not an exception as far as it's pattern of inheritance; lots of diseases are caused by double recessives like this.

I hope this cleared up some of the confusion.

I just realized that I skipped over a little introductory genetics in my last post. Apologies...

An allele is a form of a gene. Every sexually reproducing organism has two alleles for each gene, one from mom and one from dad. For many genes, multiple alleles exist (again, though, only two in each organism), but for collie eye anomaly, there are only two alleles: the good one (let's call it "A") and the bad one (let's call it "a").

For this particular disease (double recessive form of inheritance)...

A dog that is AA does not have the disease and its offspring cannot have the disease.

A dog that is Aa does not have the disease, but it's offspring can, depending on their other parent and random chance. This dog is a "carrier".

A dog that is aa has the disease and its offspring can as well, depending on the other parent and random chance. This dog is "affected".

Assuming that my mutt's parents were purebred (could be a bad assumption, but I suspect this may be the case), I know that the one that was not a BC was AA, because the bad allele is only carried by collies. The BC parent might have been AA, Aa, or aa, there's no telling. Therefore, my mutt's genotype is either AA or Aa, but not aa. To me, it doesn't matter whether her genotype is AA or Aa because I won't be breeding her, so even if she has the bad allele, she can't pass it on.

This also highlights the importance of the genetic testing done by responsible breeders. If two unaffected and untested collies are bred and their genotypes are Aa and Aa, then on average 1/4 of their offspring will have the disease, but no one would suspect they would, because neither parent has it! Kinda scary, huh? Another argument against BYB's!

Nargle: By "breeding dogs with similar traits over many generations" and acquiring "some predictability in the offspring", much genetic variation has been eliminated. That's why different dog breeds look so different despite being the same species and also why it's sometimes so difficult to tell two individuals of the same breed apart.

My top priority in selecting a dog was saving a life, so buying from a breeder was out of the question. If my top priority was the genetic health of the dog, I would have been the first one in line for the responsible breeder's pups.

The truth is getting a cross bred dog is still a risk.

There are health problems which can be inherited from only one parent.

There are also recessive health problems which require that the pup(s) get a copy of the gene from both parents. Many breeds that are commonly crossed lately carry the same problems. Pups are governed by the same genetics whether they are crossed or pure. So there is no reason to think you won't end up with unhealthy pups if you breed dog a to dog b of different breeds who carry the same problem.

There is also the issue that this isn't making for healthier dogs genetically in the long run. You are not getting rid of the bad. It is no different then breeding pure dogs in which they can carry health problems that are passed for generations until they show up in progeny. If there is a breed specific problem that is recessive and a pups dam carries this and passes it to the pup, the pup will not suffer from this since the sire did not have the gene to pass to the pup. But once you get past that F1 breeding where do you go. There is a chance even breeding to another of this cross from a different unrelated litter that the pups together will produce some offspring themselves with this problem. We can't simply breed unrelated different breeds over and over and hope to get healthier dogs by "hybrid vigor" this does not eliminate unwanted genes. Selectively breeding and culling helps to achieve this. Whether you are breeding pure bred dogs or mixed breeds.

GottaLuvMutts said:

"You can produce "hybrid vigor" in a dog (although it's not generally called this among breeders) by crossing different breeds. Please don't imply that I don't know what I'm talking about when it comes to this subject; I've taught several university-level courses which covered genetics.

Click to expand...

Then what exactly is it called? Because that (hybrid vigor) is what I've always heard it referred to, time after time for as long as I can remember, whether it crosses bloodlines within a breed or different breeds.