Genetic disorders have become a huge problem in purebred dogs. Dogs are not living as long, fertility in many breeds has declined, there are more problems caused by single mutations than ever before, and dogs suffer from much higher rates of cancer, epilepsy, and other complex problems than in the past. These are very real problems, and they fuel the public's perception that purebred dogs are unhealthy.
To understand how to fix this problem, we need to understand the cause. Let's do a mental breeding experiment.
Start with "perfect" dogs
We know from genetics that the health of a population of animals over the generations will depend on the quality of the gene pool and how it changes over time. For example, we will start with a large population of healthy individuals of a purebred dog breed. We'll make them "ideal" examples of their breed in terms of health, with all the necessary genes for physiology, anatomy, and behavior (we'll call these "dog" genes), as well as the genes for all the other traits that define type for the breed.
We have our population of terrific dogs that are perfectly equipped genetically to be exactly what we want them to be, and we would like to start a sustainable breeding program, one that would preserve not only a population of dogs for ourselves but also individuals that could establish new populations for others that would like to have the breed,. What kinds of things should we consider when we sit down to decide how we should breed to produce the next generation of dogs?
First of all, if you are interested in retaining the excellent type in your dogs, you don't want them breeding to the neighbor dogs or something else of dubious history. This would not suit your goals as a breeder, which are the preservation of health and type in your breed. You will not get dogs in the next generation like the ones in the first generation if you cross-breed; that's basic animal breeding.
If we had a population of dogs that were healthy and terrific examples of their breed, and we wanted to replicate that in the next generation, what should we do? We should make copies of all of the genes in all of the dogs of our founding populations, mix those genes up, and package them into puppies that will become the healthy dogs of the next generation.
And for the next generation? We want to do the same thing. Gather up all the genes in that first generation, replicate every single one, give them a good mix, and distribute them in the puppies of the next generation.
Of course, we might want to change something about the dogs - make the legs longer, or produce more dogs of a particular color. We can use our skills as a breeder to preferentially breed the dogs with the traits we want, using the magic of selective breeding. We make more copies of the genes we want by breeding more of the dogs with those genes, and this changes the mix of genes in the gene pool in the direction of creating dogs with the traits we want.
In a perfect world, we could continue like this forever.
But our world is not perfect. Not every gene in the current generation will end up in a new puppy, either because we have chosen not to breed some of the animals (selecting for longer legs or whatever), or simply by chance. Over time, selection and random chance will shape the nature of the gene pool, changing the frequencies of genes little by little with each generation.
If we started with founder dogs of perfect health, we want to make sure all the critical genes for health get passed from generation to generation so we continue to produce healthy dogs. But what happens if we lose a few of those, either by selection or chance? The Dalmatian, for instance, inadvertently lost a critical gene for nitrogen metabolism, with the result that the dogs suffered from the formation of urinary stones. How did breeders fix that problem? They put the critical gene back in the gene pool. They did this by crossing a Dalmatian to a breed similar in structure (the pointer), then selected the offspring that inherited that critical gene, which is identical in all dogs. It was a brilliant and simple fix. With each backcross into the breed population, with selection of the offspring that inherited that gene, the frequency of the new gene increased, the fraction of pointer genes in the gene pool dropped exponentially, and in a few generations the dogs were genetically pure Dalmatian.
Now, we have technology that might someday allow us to go in and replace that single lost gene using something called CRISPR. But it was easily restored, not with technology, but with clever breeding in a single cross.
Back to our hypothetical breeding program. If we have been randomly (by chance) or deliberately (by selection) losing genes every generation, and all the genes in our founder dogs were essential for some function, we can expect some things are not going to not work like they're supposed to. These broken things become genetic disorders - allergies, temperament problems, low infertility, cancer, kidney disease, heart failure, and any of a very long list of canine maladies.
We understand why this happens.
If we start with a population of healthy dogs and want to keep them that way, there's one critical thing we need to do - make sure every single one of the "dog" genes - the ones necessary to build a healthy dog - is passed on to dogs in the next generation, generation after generation after generation. Fiddle with the genes for type all you want, but you have to protect that original collection of "dog" genes that are necessary for building dogs that are healthy and fit to do what they were bred for.
This is the critical feature of preservation breeding. If we don't do this, we will break things.
To be fair - and realistic - it's nearly impossible to get all the genes of one generation into the next one, even if that was our sole aim as breeders. It certainly doesn't happen if we breed in a way that guarantees that some genes will be lost, and the two obvious ways this can happen is just by chance (an inescapable property of genetic inheritance) and by selection. Dogs that are bred will pass on only some of their genes; and dogs that are not bred pass on none.
If we replaced each critical "dog" gene lost like they did for the Dalmatian, we could keep a healthy population of purebred dogs forever. But if those critical genes are not replaced, there will be a deterioration in the health and function of our dogs over time. We should expect this; it's an inescapable consequence of losing genes from the gene pool each generation.
As purebred dog lovers, this is where we are. We have dogs with great genes for type - the particular traits that make each breed unique - but we have all sorts of problems with function, even to the point where breeding has become difficult in some breeds. We are hoping to cope by indentifying the critical genes that have been lost (by looking for the broken genes that have taken the normal gene's place), and we are spending millions studying the diseases that have resulted from loss of a particular critical gene. But neither of these efforts is solving the problem because we are still missing the normal copies of those critical genes.
What else could we do to solve this problem of missing genes? Why not the obvious? Why don't we just put those lost genes back, or make more copies from the dogs that have them? For some problems, it will be just a single gene as it was in the Dalmatian. In some cases, it might be multiple genes that together are necessary for some important function. But if we understand that every gene in those founder dogs played an essential role in building a healthy dog, it should be obvious that the only solution to a problem caused by missing genes is simply to put them back. In fact, this is really the only solution to the problem. Without that critical gene for nitrogen metabolism, the Dalmatian will produce urinary stones - in fact, any dog missing that gene will produce urinary stones. To solve the problem, put the normal gene back in the gene pool.
The key element of preservation breeding
If you want to have a sustainably breeding population of purebred dogs, you have to prevent the loss of the genes necessary for function from one generation to the next, or you have to replace the genes that are lost. We can't do the first if we practice selective breeding (which of course we do), so we have to solve the problem by replacement. To get back to healthy dogs, we need to restore the gene pool needed for health.
If a critical gene has been lost from one subpopulation of the breed (e.g., the dogs in the UK, or the bench lines of a retriever), it can be restored from dogs in another population that have retained that genes. In fact, there are breeding strategies that will reduce the loss of genes from a population over time by taking advantage of the ability to restore a lost gene from another population of dogs in the breed. This strategy is used by animal breeders that want to breed from a particular population of animals for many generations. It involves some clever population management and rotation of dogs among several populations that are maintained by inbreeding. In fact, this is how wild animal populations are able to persist for thousands of generations. Individuals from one population migrate to another, brining with them some of the genes that were lost over time in the new population. If populations are prevented from doing this, if they are isolated on an island for example, they eventually lose so many of the genes necessary for function that they go extinct.
For some dog breeds, critical genes might be lost from the entire gene pool, as the nitrogen metabolism gene was for Dalmatians. To replace those genes will require crossing to another breed that is selected to address the specific genetic problems most efficiently. Crossbreeding is routinely used in animal breeding to change particular traits or to restore genetic health when there has been a loss of genetic diversity. Cross-breeding is just one of several strategies breeders can use to achieve particular goals. Inbreeding, linebreeding, outcrossing, and crossbreeding are all used strategically by breeders to "shape" the gene pool so it best serves as the genetic pantry of ingredients you use in your breeding program.
The secret to producing healthy animals generation after generation - not just purebred dogs, but animals of any sort - is to maintain a gene pool that contains all the genes necessary for health. Protect the genes you have, and replace the ones that are lost, and you can breed healthy dogs forever.
ICB's online courses
Visit our Facebook Groups
ICB Institute of Canine Biology
...the latest canine news and research
ICB Breeding for the Future
...the science of animal breeding