Hybrid Vigor: Real or Assumed?

Many of us talk about “hybrid vigor”- the notion that mixed-breed dogs avoid the inherited medical maladies passed along to their purebred counterparts. Does proof exist that mixed-breed dogs are indeed healthier? An article  within the most recent edition of the Journal of the American Veterinary Medical Association helps answer this question. Titled “Prevalence of inherited disorders among mixed-breed and purebred dogs: 27,254 cases (1995-2010)”, this study determines the proportion of mixed-breed and purebred dogs with 24 common genetic disorders including four types of cancer, orthopedic issues, birth defects, bloat, hormonal imbalances, eye issues, allergic dermatitis, and epilepsy.

The Results

Compared to their purebred counterparts, mixed-breed dogs were more susceptible to only one inherited disorder- tearing of the cranial cruciate ligament (the main stabilizing ligament within the knee).

Purebred dogs were more likely to develop ten specific inherited disorders including:

Aortic stenosis (a birth defect within the heart)- Breeds most affected: Newfoundland, Boxer, Bull Terrier, Irish Terrier, Bouvier des Flandres

Dilated cardiomyopathy (disease of the heart muscle resulting in heart failure)- Breeds most affected: Doberman Pinscher, Great Dane, Neapolitan Mastiff, Irish Wolfhound, Saluki

Hypothyroidism (inadequate production of thyroid hormone)- Breeds most affected: Giant Schnauzer, Irish Setter, Keeshond, Bouvier des Flandres, Doberman Pinscher

Elbow dysplasia (malformation within the elbow joint)- Breeds most affected: Bernese Mountain Dog, Newfoundland, Mastiff, Rottweiler, Anatolian Shepherd

Intervertebral disk disease (slipped disk)- Breeds most affected: Dachshund, French Bulldog, Pekingese, Pembroke Welsh Corgi, Doberman Pinscher

Allergic dermatitis (skin allergies)- Breeds most affected: West Highland White Terrier, Coonhound, Wirehaired Fox Terrier, Cairn Terrier, Tibetan Terrier

Bloat (gastric torsion, twisting of the stomach)- Breeds most affected: Saint Bernard, Irish Setter, Bloodhound, Great Dane, Irish Wolfhound

Cataracts– Breeds most affected: Silky Terrier, Miniature Poodle, Brussels Griffon, Boston Terrier, Tibetan Terrier

Epilepsy– Breeds most affected: Catahoula Leopard Dog, Beagle, Schipperke, Papillon, Standard Poodle

Portosystemic shunt (a birth defect causing shunting of blood around rather than through the liver): Breeds most affected: Yorkshire Terrier, Norwich Terrier, Pug, Maltese, Havanese


No differences in disease incidence between the mixed-breed and purebred groups were found for 13 different disorders including:

The cancers evaluated (hemangiosarcoma, lymphoma, mast cell tumor, osteosarcoma)

Hypertrophic cardiomyopathy (a disease of the heart muscle)

Cardiac birth defects (mitral valve dysplasia, patent ductus arteriosus, ventricular septal defect)

Hip dysplasia

Patellar luxation (displacement of the knee cap)

Addison’s disease (a hormonal imbalance)

Cushing’s disease (a hormonal imbalance)

Lens luxation (displacement of the lens within the eye)



The authors of this study surmised that genetic mutations resulting in inherited defects may have developed at different times during the evolution of dogs. For example, a mutation introduced very early on into the canine genome (closer in time to the wolf progenitor) would have been spread throughout the entire dog population, purebred and hybrid alike. For disorders affecting purebred dogs in higher proportions, the underlying genetic defect may have occurred more recently, after the gene pools for a particular breed (or related breeds) evolved.

It is possible that the same genetic traits that predisposed to domestication are chromosomally connected to the genetic traits that predisposed to the disorders shared equally by purebred and mixed-breed dogs.

It is possible that the hard wiring for some of the genetic disorders is chromosomally linked to the DNA that determines dog size rather than dog breed. While patellar luxations and lens luxations occur without breed specificity, they do occur primarily in small dogs. Conversely, cancers are more likely to occur in large breed dogs.

Lastly, the results of this research may have been influenced by the hospital population (study performed at UC Davis). For example, we know that Standard Poodles are genetically predisposed to Addison’s disease, yet the results of this study do not demonstrate this. If the study population contained an overabundance of Standard Poodles (evaluated for a wide array of diseases), it is possible that their specific predilection for Addison’s disease may have been “diluted out”.

So now what do we have to say about hybrid vigor? Clearly, for some inherited diseases, this theory fits. For others, it is inapplicable.

Do the prospects for future diseases influence which dogs you choose to adopt?

Best wishes,

Nancy Kay, DVM

Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
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Please visit http://www.speakingforspot.com to read excerpts from Speaking for Spot and Your Dog’s Best Health.   There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at www.speakingforspot.com, Amazon.com, local bookstores, and your favorite online book seller.

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15 Comments on “Hybrid Vigor: Real or Assumed?

  1. Forgot to comment on this past post…………Katie is a mix-breed with an auto-immune. Not sure that the DNA tests are always accurate? I’ve heard different results from different folks(Dutchies, GSD’s,etc). It would be nice to have a reliable DNA test done on her to add her numbers to the statistics. There is a kennel in another southern state with similar looking dogs………

    The Other Nancy

  2. This study does not address hybrid vigor, or heterosis, which is more that the simple susceptibility (or lack thereof) to certain genetic disorders, which is what this study looked at. Unsurprisingly, purebred dogs were at increased risk for some but not all disorders, and mixed breed dogs were at increased risk for being hit by a car, or having a CCL tear, which is most closely associated with being spayed or neutered.

    The Press Release for this study did a very poor job of describing the results of the study, and also unsurprisingly, some people have jumped to say “aha, hybrid vigor isn’t real!” even though it was not addressed, at all, in the study. To those who continue to deny the reality of heterosis, please do your research, which has shown the benefits of a decreased COI and/or heterosis for every species studied, including dogs.

  3. Bunch of Bunk!
    How many mixed breeds are health tested generation after generation and registered in health databases such as OFA and CERF? How many of the dogs in their pedigree (do you know your mixed breed’s pedigree?)
    Cruciate ligaments are likely injury from agility, disc sport, flyball, etc – which mixed breeds participate in.
    Don’t get me wrong, purebreds have health issues – but like anything, the more you investigate and record, the more you find. But look at the list of disorders – how many mixes are labs, gsds, collies, poodles, etc. bred into? If you tracked their offspring – which most will not have because most mixed breeds are neutered, I think your hybrid vigor would wilt and fade.
    You will never convince me.

  4. In many yeras I lived with only “hybrids” (whom I call “random breeds”) with the exception of one pure-bred Beagle. All were Healthy both physically and mentally, stronger, more educable, different, whimsical, unique, affectionate and all lived to ripe old ages, saome large, some medium. Two of the large ones lived to be 18 with good quality of life. All, that is except the little Beagle. She had more health issues (both physical and emotional) than any of the others and I lost her at age 9 to severe illness.

    I never buy, only adopt. I always look for my good friends, the random breeds and nature has never let me down. Obviously they are not inbred or even line bred, they own the good genes and characteristics of “accidental breeding” of whoever was involved in the “l’affaire du’coeur” at the time.

  5. Hybrid Vigor does exist and it is not just about the lack of disease. Originally used in livestock it was about offspring of dissimilar parents being “better” than those parents. In cattle and hogs it was about meat production, in horses about athleticism and speed. These kinds of traits are most likely highly poly-genetic and cannot be broken down by simple Mendelian inheritance.

  6. We can, to some extent, have our cake and eat it too. There are folks out there working to find ways to make “purebreds” more viable over the long run. The Backcross Project for Dalmatians is an example of how it has been accomplished for one persistent problem within the breed:

    And Dr. Bragg’s multipart article offers many other ideas of how this might be done:

    Of course if these dogs are to be registered and shown, there must be acceptance by the breed groups and the registering bodies (AKC, UKC, etc.). That and an improvement in the information available to breeders through genetic testing and their technical understanding of breed genetics can make a difference. Simply mating two pure-bred dogs to produce a litter is not sufficient to save our much loved breeds.

  7. the discussion and comparisons of diseases in purebred or mixed breed dogs can be interesting and valuable. In this simple out line, there are far too many weaknesses and ‘holes’ to draw any valid conclusions.

    HOWEVER, the the most egregious error is calling dogs of mixed breeds ‘hybrids’. This is a fallacy , and is scientifically false.

    Different breeds of dogs are still same SPECIES. A hybrid is a combination of two different albeit compatible species.

    Making this basic error throws any further credibility into disrepute.

  8. I have purebred dogs for a reason – I know the parents, I know the health issues, and I also only get a dog from a breeder who does all of the available health clearances. And I do all of the clearances on the dogs I get. My last three dogs, a Labrador, and 2 Gordon Setters (mother and son) lived to 15, 16, 13 respectively. None of them inherited the common disorders of their breed – hip dysplasia/bloat/thyroid issues. At the present time my two Gordons have all of their clearances and one, from a foreign breeder, is a carrier for one disorder.

    The one mixed bred dog I had also lived a long life.

    I work for my vet, and the mixed bred dogs show up with the same issues as the purebreds do.

  9. Thank you for bringing our attention to this paper. I have written Dr. Oberbauer for a reprint.

    I dislike the term, hybrid vigor, so I will be interested in their definition of it.

    Also, I will be interested to see if they use the term, proof/prove, since statisticians don’t use it as I recall. We don’t actually prove anything in science, we either support (the null hypothesis) or disprove it (but that is old terminology).

    Skye Anderson, MS (Genetics)

  10. I love Cardigan Welsh Corgis. They have a wonderful disposition and are a joy to live with. However, I can’t ignore the ticking time bomb that is IVDD. I definitely worry that one of my boys will rupture/slip a disk. I do what I can by keeping the slim and fit, but that doesn’t give any guarantees.

    One thing that is good is the breed isn’t very popular and the vast majority of the breeders are working hard to eliminate the diseases that can be tested for, such as PRA and DM. It is possible now to purchase a puppy you know will not come down with DM, not something as easily said for the much more popular Pembroke Welsh Corgi.

    All that being said, I am not sure my next pup will be another CWC. The back issues weigh heavily on me.

  11. Absolutely it influences my decisions. When I choose to purchase a dog from a breeder, I make sure that breeder is one who tests their breeding stock and follows the current breeding protocols to reduce amount of inherited disease from their lines. In the case of the Cavalier King Charles Spaniel, that would be mitral valve disease and syringomyelia. If I’m adopting a pet, I would rather get a purebred because then I know what genetic conditions to watch out for, and can be more proactive in helping them stay healthy.

    As for a “mutt” (Most Unwavering Treasured Teammate), which is what “designer” breeds are, I don’t believe there is any significant evidence that shows them to be healthier. You are just as likely to get the worst of both breeds as the best of both breeds.

  12. After nursing a dearly beloved dog for a couple of years through congestive heart failure, to its inevitable end, I looked long and hard for a healthy small dog. Not as easy as it sound! I finally adopted a 2-yo Maltese/poodle who now at age 3, continues to check out as healthy and strong on her recent vet visit. Someday I expect I’ll be nursing her too, but at least she is starting out as a strong adult and has every expectation of living a healthy active life. My heart goes out to all of those who are not as “genetically lucky” but I have no patience for the irresponsible breeding practices that have engineered unhealthy, short lived dogs.

  13. I’m not convinced. None of the diseases of the cocker spaniel are in the list, so cocker spaniels aren’t in the list either. I would think that if they want to know whether the genome closest to the wolf is least messed up they should study the breeds most closely to the wolf as their next study – do GSDs have more or less disease than GSD mixes?

    Also, they don’t factor in dogs from puppy mills and backyard breeders vs. dogs from breeders that screen for genetic disorders. Someone who has paid $2,000 or more for a pedigree like that would show up in a high-cost clinic for other diseases just because they have money. Likewise, if you have paid thousands of dollars for a champion bloodline, would you let it run loose and mate with the neighborhood mutt? And puppy mills and backyard breeder dogs would come from inbred lines with no research, so for dogs more commonly bred that way you’d expect more genetic diseases. But if you paid $150 for your shih-tzu from a breeder in the local paper, are you going to spend thousands of dollars for cancer treatment?

    Transitional cell carcinoma (bladder cancer) is associated with the Westie but my dog was a white shih-tzu mix (mostly shih-tzu). I’m surprised they didn’t include that cancer since it’s so well studied.

  14. I am a dane girl through and through. Despite the severe medical issues this breed faces, they are my true love and I will handle whatever is thrown at us. In my own danes, I have dealt with wobbler’s syndrome, addison’s disease, diabetes, acl ruptures, slipped disks, OCD, and cancer.

  15. So the answer is yes, there is such a thing as hybrid vigor. Purebreds are susceptible to many more conditions likely due to the inherent inbreeding that exists. For so many reasons, I always adopt mixed-breed dogs rather than buy purebreds. I enjoy their diversity in looks, their general longevity and their improved mental health, which while not yet proven, I believe to be true.