If you have reached this link via the Bark magazine blog, you should understand that this article is in no way a defense of closed registries. Rather, it is a statement that higher rates of genetic disease are an almost unavoidable consequence of closed populations, and thus the debate between which is healthier, pure- or mixed breed dogs, is just plain stupid, and purebred dog breeders need to own up to that fact. If you’d like to know more about the genetic consequences of closed registries, please see my articles Closed Registries: Dogs in the Handbasket to Hell, Part I and Part II.
The proper question would be:
What are the risks of (genetic) disease associated with this breed, and how do they compare with other breeds and mixed breeds as populations?
Because this question is, well, loaded in regards to dogs, we will consider it first in humans.
Increased or decreased risk of certain genetic diseases is well documented in humans. A well-known example would be Tay-Sachs disease, a neurological disease caused by a simple recessive (the child must inherit two copies of the gene to be affected, one from each parent.) Anyone can be a carrier of Tay-Sachs, but it is most common in the Ashkenazi Jewish population, with about one out of every twenty-seven people carrying the gene.
Why would this gene be so common in one population of people? Founder effect. In this case, it would be due to a population bottleneck, or reduction of the population to relatively few individuals, from which a larger population grew. Founder effect also occurs when a small population breaks off from a larger one. Like the Hutterites, a branch of Anabaptists that can be traced to a group of less than 100 individuals that migrated to North America in the 1800s. Almost one out ten Hutterites is a carrier of Bowen-Conradi syndrome, a disease that causes failure to thrive and various deformities in infants. Bowen-Conradi syndrome is considered a rare disease…unless you are a member of the Hutterite community.
Other examples of founder effect on genetic disorders would be Huntington’s disease in San Luis, Venezuela, a plethora of heritable diseases in the Amish and Mennonite communities, and the Finnish Heritage Diseases, a group of disorders that are rare elsewhere but more common in Finns due to their small founder population.
Why do some of these genetic disorders remain at such high frequencies in some populations? Many communities are genetically isolated, either through location (like on an island or remote village,) or through culture (religion.) These communities typically produce children with other members of their community, and thus are reproducing with people related to them, which would increase homozygosity in the offspring. This also keeps gene frequencies high. Producing children with unrelated outsiders would result in a continuous influx of new gene variants, changing the frequency of deleterious genes. Small, closed (or mostly closed) populations are especially vulnerable to genetic drift, or changing gene frequencies (spread of deleterious genes.)
(Something not usually seen in humans but common in dogs is the popular sire effect, where one male dog may produce many, many offspring, and alter the genetic signature of a breed or a large portion thereof. If this sire is carrying a gene for a genetic disorder, it can spread through a population rather easily.)
Not all genetic disorders are inherited in a simple dominant or recessive manner. Some dominants have incomplete penetrance, some are linked to either sex. Another type of inheritance is complex or multifactorial. This type of disorder commonly runs in families but the way it is inherited is unclear, and there is often an environmental component.
Why might some populations have higher or lower frequencies of disease?
- Larger number of diverse founders (lots of founders, founders not related to each other)
- Lower rate of inbreeding (mates as distantly related as possible)
- Occasional influx of unrelated individuals (outcrossing)
- LUCK! (low genetic load in founders, drift in favor of non-carriers, disease kills individuals before they can reproduce)
Okay, now that we have established that in small, closed populations of humans it can be typical for there to be higher rates of certain genetic disorders, and that this can be caused by founder effects, and maintained by breeding within the population instead of outbreeding, let us revisit the original question regarding dogs:
What are the risks of (genetic) disease associated with this breed, and how does this risk compare with other breeds and mixed breeds as populations?
Can we look at individual populations of dogs (single breeds) and discern rates of disorders? Yes. Can we look at mixed breeds and see the rates of genetic disorders in that population as compared to other populations? Yes. Can we look at ‘dogs’ as a population (all dogs) and compare rates of genetic disorders? Yes.
Why then do we accept that we can do this in humans, and accept the results (some populations have higher or lower rates of disease and disorders than other populations) but we adamantly deny this kind of data when it comes to comparing dog populations?
Just stupid, I guess.
Let’s look at some dog studies, shall we?
The breeds at significantly higher risk of developing IBD compared with mixed-breed dogs consisted of weimaraner (odds ratio [OR]=3.6797, 95 per cent confidence interval [CI]=2.0167 to 6.7141, P<0.0001), rottweiler (OR=2.9697, 95 per cent CI=1.7569 to 5.0196, P<0.0001), German shepherddog (GSD) (OR=2.4101, 95 per cent CI=1.5826 to 3.36705, P<0.0001), border collie (OR=1.9936, 95 per cent CI=1.1655 to 3.4101, P=0.0118) and boxer (OR=1.6961, 95 per cent CI=1.0441 to 2.755, P=0.0328). This study demonstrates for the first time canine breeds in the south-eastern UK that are highly susceptible to developing IBD. Identification of such breeds may allow for a more focused investigation of genetic mutations associated with canine IBD.
This tells us that Weimaraners, Rotties, GSDs, and Border Collies are at higher risk of developing IBD than mixed breed dogs in the Southeastern UK. Does this mean that mixed breed dogs are healthier than purebred dogs? No. Does this mean that purebreds are healthier than mixed breed dogs? No. It means that “Weimaraners, Rotties, GSDs, and Border Collies are at higher risk of developing IBD than mixed breed dogs,” and the fact that these breeds are over-represented in regards to being affected by IBD indicates there may be a hereditary, genetic component to the disease.
A human being of Ashkenazi Jewish heritage has a higher risk of being a carrier of Tay-Sachs disease than an individual who is not of that heritage. A German Shepherd Dog has a higher risk of being affected by IBD than a dog of mixed breed heritage. Why do we accept the former but the latter makes us start hand-waving and shouting?
Just stupid, I guess.
Let’s play the game again.
RESULTS: Breed period prevalence of OSA was highest for Greyhounds (21/339 [6.2%]), followed by Rottweilers (51/969 [5.3%]) and Great Danes (13/297 [4.4%]); all 21 Greyhounds with OSA were identified as having retired from racing. Sex was not identified as a risk factor for OSA in these breeds, but in all 3 breeds, risk of OSA increased with age. Greyhounds were significantly older at the time of OSA diagnosis (mean, 9.9 years) than were Rottweilers (8.3 years) and Great Danes (7.8 years). Rottweilers and Great Danes were more likely to have OSA involving the forelimbs than the hind limbs. The most frequent lesion sites for all 3 breeds were the proximal end of the humerus and distal end of the radius. The proximal end of the femur was also a common site for the Greyhounds.
CONCLUSIONS AND CLINICAL RELEVANCE: Results of the present study suggested that Greyhounds, Rottweilers, and Great Danes had an increased risk of developing OSA, compared with mixed-breed dogs.
Great Danes have a greater risk of developing osteosarcoma, bone cancer, than mixed breed dogs. Also, Great Danes are more likely to develop osteosarcoma in their front legs than in their hind legs.
Does this mean that mixed breeds are healthier than Great Danes? NO. It means that if you own a Great Dane you should be aware that they have a breed disposition towards osteosarcoma and you should watch your dog for signs of limping or other discomfort, especially as it gets older. If you are a breeder you may wish to think twice about breeding from or to a dog which has a familial history of osteosarcoma.
Exactly why is ‘risk factor’ is so hard to understand?
Just stupid, I guess.
Let’s spin the wheel one more time.
PROCEDURE: 165 breeds (including a mixed-breed category) were represented in the hospital population. Breed-specific expected numbers of dogswith DM were calculated by multiplying the proportion of all dogs admitted to the hospital that were determined to have DM during the study period by the breed-specific totals during the study period. Breeds or breed groups evaluated in the analysis (n = 20) were restricted to those that had a combined observed and expected count > 5 to document breeds at low and high risk for developing DM. Proportionate changes in the risk of developing DM by breed were calculated and presented using exact odds ratios, 95% confidence intervals, and P values. Mixed-breed dogs were chosen as the reference breed.
RESULTS: Samoyeds, Miniature Schnauzers, Miniature Poodles, Pugs, and Toy Poodles were at high risk for developing DM. Dog breeds found to be at low risk for developing DM were German Shepherd Dog, Golden Retriever, and American Pit Bull Terrier.
CONCLUSION AND CLINICAL RELEVANCE: The finding that certain dog breeds are at low or high risk for developing DM suggests that some genetic defects may predispose dogs to development of DM, whereas other genetic factors may protect dogs from development of DM.
‘Reference breed’ means that mixed breeds were chosen as a population representing the ‘normal or typical’ rates of diabetes mellitus in dogs. Since mixed breeds can be presumed to have diverse genetic profiles, this can tell us whether a higher than normal rate of disease may be hereditary in nature, and allow us to look for clues in the genome as to how DM may be inherited in specific high risk breeds. In this study, Samoyeds, Miniature Schnauzers, Miniature Poodles, Pugs, and Toy Poodles were at higher risk than mixed breed dogs for developing DM. But, German Shepherd Dogs, Golden Retrievers, and American Pit Bull Terriers were at lower risk than mixed breed dogs.
Once again: does this mean that mixed breed dogs are healthier than purebred dogs? No. It means that mixed breeds are typically at lower risk for DM than purebred dogs, and that some breeds have a higher risk profile, and some have a lower risk profile, than mixed breeds.
Just like in human populations. And, just like in human populations, heterogeneous, genetically diverse populations typically have lower rates of heritable disease than homogeneous populations. In other words, mixed breed dogs are genetically more diverse than purebreds, and thus two disease causing genes are less likely to occur in the same dog. With more complex inheritance, purebred dogs are more likely to have ‘runs of homozygosity’ (long stretches of identical DNA, due to founder effects) than mixed breed dogs, and therefore more likely to have higher risk of diseases caused by polygenetic or complex inheritance.
There are a plethora of studies showing risks factors in purebred dogs compared to mixed breeds. Often, but not always, purebreds have a higher risk factor. Being a ‘breed’ means predictability, both in appearance and behavior, and that means a more homogeneous genetic population. A higher risk for certain disorders is a natural consequence of founder effects and closed populations. It is not something to be ashamed of, and it is not something to deny. It is something to be aware of, to work towards changing, and to educate the public about.
This is not rocket science. It is not hard to understand. Studies and good health surveys can tell us what the risks are for our breeds. This helps us make breeding decisions and it helps us to educate puppy buyers. Being honest about those risks helps us to mitigate them. Surveys, studies, databases, tests and even outcrossing to another breed are simply tools to help us deal with genetic disorders in our dogs.
No population is free of disease. A higher frequency of some diseases is a tradeoff for the predictability of being ‘purebred.’ Changing the frequency of disease in a population is possible, and this is what breeders are attempting to do when they conduct surveys and studies and do health tests and look for disease genes and choose certain dogs for breeding while passing others over. It has nothing to do with which kind of dogs are ‘healthier’ and everything to do with trying to decrease disease frequency in our own dogs.
Turning the discussion of dog health into a pissing contest between purebreds and mixed breeds and their fan clubs is counterproductive. Delving deeper into purebred health, which is easier due to their genetic consistency, helps all dogs, and humans. Why, then, does any discussion about dog health, diversity, disease rates, and genetics seem to devolve into “are mixed breeds healthier than purebreds?”
Just stupid, I guess.
References and resources
Disorders in human populations
- Tay-Sachs Disease
- Founder effect in Ashkenazi Jews
- Bowen-Conradi disease
- Bowen-Conradi disease in Hutterites
- Disorders in Amish and Mennonites (pdf)
- Huntington’s disease in Venezuela
- Finnish Disease Heritage
Genetic drift and gene frequency in small populations
- Small population size effects
- Genetic drift and the founder effect
- Genes in populations
- Jews and increased risk
Studies showing risk in purebred relative to mixed breed dogs (lots more out there, knock yourself out)
- Canine breeds at high risk of developing inflammatory bowel disease in the south-eastern UK
- Prevalence of and intrinsic risk factors for appendicular osteosarcoma in dogs: 179 cases (1996-2005)
- Analysis of 14,008 uroliths from dogs in the UK over a 10-year period
- Association of breed with the diagnosis of congenital portosystemic shunts in dogs: 2,400 cases (1980-2002)
- Breed risk of pyometra in insured dogs in Sweden
- Breed distribution of dogs with diabetes mellitus admitted to a tertiary care facility
- Risk factors for acquired myasthenia gravis in dogs: 1,154 cases (1991-1995)
- Risk factors associated with parvovirus enteritis in dogs: 283 cases (1982-1991)
- Predisposition for primary hyperlipidemia in Miniature Schnauzers and Shetland sheepdogs as compared to other canine breeds
- Effects of risk factors and prophylactic treatment on primary glaucoma in the dog
- Epidemiologic features of canine hypothyroidism