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Copyright 2002-2016 Dr. Jennie Chen. All images and articles are copyrighted.  Unauthorized use is strictly Prohibited.

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Introductory disclaimers:

  • This article is to clarify the Myth of Hybrid Vigor. It is a myth like the Yeti and the Loch Ness Monster. It is something that people say exists, but there is no real scientific evidence. I have not been presented with any scientifically sound proof at cross breeds are any healthier than responsibly bred dogs.
    • If you start your next thought with, "But I know of a designer dog that isn't unhealthy." or "I had a healthy mutt when I was a kid.", stop right there. You're basing your claim on anecdotal evidence. You'll be the butt of scientific jokes. People will mumble, "N of 1." and giggle in your general directional. N of 1 means that you are basing your beliefs on a single example instead of studying a representative population.
    • There are some unsavory alternatives presented many times in other places on why mixed breeds might not be diagnosed with disorders as often as purebreds. In my personal experience, I see obvious structural disorders and symptoms of diseases in mixed breeds that don't even cause their owners to bat and eye.
  • *Do note that I am not using the term hybrid vigor in the scientific definition. I'm addressing hybrid vigor as the marketing term in puppy sales.* Here's some definitions of responsible and irresponsible breeders. Keep in that many irresponsible breeders these days have stepped up their marketing by registering their animals with unscrupulous companies and are charging more than responsible breeders. The unscrupulous registries can be likened to businesses online that sell degrees.
  • You should read this article if you're asking yourself, "Should I buy a designer dog?" "The craigslist ad said that this dog is hypoallergenic. Should I trust the ad?" "Designer breeds are so healthy and cute! Can I go wrong?"
  • You should probably figure out if you really want a dog. Then you should decide what type of dog works with your lifestyle. Then you should figure out where to get said dog that fits your lifestyle. My recommendations for the source is : 1. shelter 2. reputable rescue 2. reputable and responsible breeder.
  • Please do more research on a living creature that will share its life with you. Most people do less research on getting a dog than they do on what phone to purchase. And often that phone is replaced in a few years.

Hybrid vigor in dogs does NOT exist in practice.  It only exists in theory - much like Communism.  Health problems cannot be avoided by mixing breeds - they can only be reduced through responsible breeding.  Additionally, purebreds are not unhealthy due to inbreeding.  That is also a myth.  Unhealthy purebreds are a result or poor and irresponsible breeding.  Mixed breeds are NOT any healthier than pure bred dogs.  A mixed breed dog can be just as unhealthy as any poorly bred pure breed dog. There is no guarantee of health - mixed or purebred. Run from anyone who tells you otherwise. See links on so you think you want a dog for more info. 

Unfortunately, most purebreds that you see in pet stores, on the side of the road, in the Wal-mart parking lot, or advertised on some puppy buying website are in terrible shape - thus the notion that purebred dogs are unhealthy due to inbreeding.  They have poor structure.  They are in poor health.  Those are the ones that people are spending a small fortune on.  Unfortunately, that's the general public's view of purebred dogs - just the poorly bred ones.  And more unfortunately, the general public is only exposed to irresponsible breeders, rarely do they encounter the responsible ones.   I would say that 98% of breeders are irresponsible across the board.  That only leaves a handful of the good ones - the ones that are actually improving the breed.  See links on so you think you want a dog for information on responsible versus irresponsible breeding. 

The main concern with hybrid vigor is that is tends to give the public the notion that they can breed their dogs with anything and anyone and it will automatically be "healthier."  This cannot be further from the truth.  Two unhealthy dogs do not produce healthy puppies (*unless you are really lucky. In that case, buy me some lottery tickets).  The types of "breeders" (term used loosely) that push the "hybrid vigor" myth are the types that get poor purebred quality dogs, then produce poor quality mixed breeds.  No responsible breeder would do that.  And there is not a single breed club in America that endorses that practice nor do I know of a single vet that endorses that practice.  The people who do that are only after the mighty dollar. *There may be a handful of reasons to cross dog breeds for a specific purpose by responsible breeders, but I have yet to meet one.

Hybrid or cross-breed vigor can only work in theory if two healthy dogs (free of genetic disorders, sound structure, etc...) that are not affected NOR carriers of the disease are bred.  Some might argue that hybrid vigor doesn't even occur in the same species, rather only in cross-species mating.  However, this is not what happens in practice.  What happens in practice is that said "breeder" will buy some dogs, breed them, sell puppies as designer dogs.  That's actually what most of them do with purebreds as well.  They are essentially outcrossing two unhealthy gene pools.  They do NOT get health certificates on their dogs.  Many of the hereditary diseases are asymptomatic.  They do NOT know the dog's pedigree, thus not know if the dog is a carrier or not.  They are basically going into it blind as a bat with only $$$ in mind.  Hybrid vigor in dogs now is really just another marketing ploy. It doesn't take but a quick Google search to reveal the practice.

They are also making the mistake that all diseases are randomly distributed. They ASSUME that due to random distribution, acquiring a dog and NOT doing health clearances, will mean that their dog should have the same risks of producing diseases as any other dog. Wrong assumption. Genes are not randomly distributed - genes are inherited from the sire and damn. Puppies cannot have genes that the parents DON'T have. It isn't possible. Puppies can only inherit the genes the parents do have - half from each parent (Though the genes are not randomly distributed. Some genes are more heritable than others. Many people call highly heritable phenotypes "stamping."*See below for examples.). If the parents have unhealthy genes, then it isn't too hard to see what the puppies will inherit. This is taught in a basic probability class. What happens when you mix two bad sets of genes? You get puppies with bad genes! Duh! Two unhealthy parents don't make one healthy puppy, unless it was a miracle. And seeing how I haven't won the lottery, miracles aren't all that common.

Having a small gene pool has nothing to do with health problems.  Some of the healthiest dog breeds I know have small gene pools that have been well-maintained by breeders who have been dedicated on improving the breed in health, type, temperament, and structure. I should also add that a small gene pool is NOT the same as inbreeding or line breeding as defined as having no common ancestors for three generations. Here's a link about inbreeding coefficients.  One can have a very small gene pool and still have a calculated R of close to zero.  This coefficient also depends on how many generations you go back in the calculations.  

Having a gene pool full of genetic disorders - large or small - and breeding irresponsibly is a problem.  Many breed clubs actually have programs in place that track the health of the breed.  There's always and the chic registry.  Many breed clubs also do a mass survey to owners every 3-5 years to track trends in health.  Then it is up to the breeders to try to continue breeding to reduce the frequency of those problems. Select dogs to breed who are not carriers or exhibitors of genetic defects. If you do decide to breed to a carrier, do so carefully. But remember, those are the responsible ones who invest time and effort into improving the health of their breed.  The irresponsible people probably don't even know that a breed club exists.  

  • I made changes to this article to make it clear that I am not pushing for or against mixed breeds or pure bred dogs. I am advocating that one should not blindly purchase a puppy that is a designer dog or because the "breeder" insists that the dog has hybrid vigor.
  • Bellumori et al (2013) claimed that mixed breeds breeds were healthier than purebreds, however in typical academic fashion, the conclusion far overstepped what the data could demonstrate. The study design is NOT convincing to me as there were quite a few methodological flaws. I would not personally cite that study as scientific evidence. Study it yourself here. Keep in mind that published articles often don't pass scientific scrutiny.
  • Know that theory and practice are divorced from each other. What you might read in a theoretical paper or a study generated in a laboratory setting can be worlds away from reality. Many things work on paper and don't translate into what actually happens. There is no guarantee when it comes to genetics. There is only reducing or increasing risk.
  • After reading several scientific articles, it is pretty clear that there are serious methodological problems with the studies. Instead of having you read and digest each of the articles when you come across a hybrid vigor marketer or extremist, here's some of the reasons why those studies do not mirror reality.
    • Many of the studies rely on samples from vet schools that show a large percentage of purebreds over mixed breeds. The question to ask is, "Why are there more purebreds in vet school clinics than there are mixed breeds when there are more mixed breeds than there are purebred dogs?" The possible answers to that question are quite unsavory, so I won't go there. However, it remains that the sample in vet clinic studies does NOT represent reality.
    • Of all the studies I read, dogs were segmented into mixed breed and purebred as if there were only two groups that were homogenous within groups. This is like lumping all Asian into a single group - which is also offensive. Within purebred dogs, there are different types of breeders producing very different quality dogs. To apply a single assumption to both groups is a serious, serious methodological mistake. I can certainly see how academics would make that mistake as they are not familiar with the practice or subcultures of dog breeders. Again, theory and practice are divorced from each other.
    • Not a single study actually examined inbreeding coefficients. To scientifically provide evidence that homogeneity of gene pools is detrimental to health, inbreeding coefficients need to be assigned as the dependent measure in an experimental design. The lack of measuring inbreeding coefficient means that the results of said studies are meaningless to responsible purebred do breeders and owners. Also, you can cut data in any fashion to have it show you what you want it to show. Certainly, Bellumori (2013) did just that.
    • Speaking of Bellumori (2013), the way the analyses were displayed was quite deceiving. All the mixed breed dogs were groups together with no segmenting of the predominant breed. It is obvious and expected that if you look for breed specific genetic disorders and compared it to mixed breeds that do not have any of those breeds as ancestors, you will see that the mixed breed doesn't have that breed specific genetic disorder. An example of this would be to look for common boxer genetic health problems in boxers from irresponsible breeders and a maltese mixes. The results should not surprise anyone. Bellumori (2013) did not give the total number of subjects in the purebred categories that were displayed in the graphs, and it does damage the scientific integrity of the study. Did I mention that not all scientists agree?
      • Should the question of "are purebreds healthier than purebreds" become a priority for a researcher again, I suggest the following study design and recruiting. Recruit all dogs from the same responsible purebred breeders. As said responsible breeders to produce two litters - a purebred litter of their choosing and a mixed breed litter from from the same parents (with some other breed of course). This should be repeated until there are at least 200 individuals dogs of each purebred and mixed breed for each breed to be studied. You could add another condition that another set of dogs be recruited from irresponsible breeders as well to make it a 4x4xbreed design. Make absolutely certain that food, lifestyle, training, and climate for all these dogs is exactly the same. Now a researcher can get a clearer picture of between and across breeds and breeder types results along with inbreeding coefficient impacts. I haven't seen a study like that yet.
    • If you're looking for good examples of how academic research meets real life practice to actually make an impact, I suggest a few episodes of Freakanomics Podcasts. I also love that the show is not emotionally committed to either outcome, and they are extremely outcome oriented. Emotions and research about dogs are often emotionally fueled and guided, which can be detrimental.
  • I am not making breeding recommendations. I am aware of genetic diversity and the dangers of homogeneity in organism populations - within and between species and breeds. I'm not addressing those issues nor do they impact the outcome of my article. Many responsible breeders are already incorporating this into their program by studying the inbreeding coefficients of their planned litters.
  • I do believe that applying poor science to responsible breeders is not an effective approach. Responsible breeders aren't the ones overpopulating shelters nor are they the ones willfully producing dogs with poor health. If you would like to be effective and efficient, the irresponsible breeders are your target audience. Go visit them in a Walmart parking lot or find them on a website full of marketing fluff but no evidence of dedication to a breed's type, temperament, structure, or health.
  • The human health literature suggests that up to 90% of your long-term health outcomes are attributed to lifestyle. Humans who live an active, peaceful, social, purposeful, and nutrient rich lifestyle tend to live longer despite their genetic makeup. Once you have a dog in your hands, you're likely better off focusing on providing your dog with proper training, nutrition, confidence, mental stimulation, and a job. Worrying and debating with others about your dogs' genetic makeup won't make you or your dogs any healthier.
  • Here's some other articles to read. They are not scientifically based, but interesting insights to read nonetheless.

Copyright 2002-15 Jennie Chen, PhD. All images and articles are copyrighted.  Unauthorized use is strictly Prohibited.























Additionally, many diseases can be multigenetic or plietropic, but there hasn't been a documented one in canines that is protagonistic plietropic.  Multigenetic means it multiple genes have been linked to a disease.  An example would be canine hip dysplasia.  Plietropic means that the gene has multiple effects.  An example of this in humans would be PKU (phenylketonuria) that can cause mental retardation, reduced hair, and skin pigmentation.  An example of this in canines are colored linked diseases.  There are many diseases in canines that have been linked to light colored coat or lack of pigmentation.  Most of the time, what we see in dogs and humans is antogonistic plietropic.  Antagonistic pleiotropy refers to the expression of a gene resulting in multiple competing effects, some beneficial but others detrimental to the organism.  This flies in the face of people who say that the bigger the gene pool, the fewer risk of diseases.  Just not true.