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- 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
- 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
- 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
- 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
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.
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
www.offa.org 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
- 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
- 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.
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
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.