Does Hybrid Vigor Exist in Labradoodles and
Goldendoodles?

Good question.  Before addressing it I’d like to address the somewhat
smug assertion by some that hybrid vigor cannot possibly exist in
Labradoodles, Goldendoodles,  and North American Retrievers (NAR)
because Labradoodles, NAR, and Goldendoodles are not hybrids. The
premise for this assertion is a very restricted definition of the term
hybrid. This is akin to claiming that the opening in the front of your
trousers cannot possibly be called a fly, because the first definition in
the dictionary says a fly is a winged insect.  Yes, one definition of a
hybrid is a cross between species, and another definition is a cross
between populations.  It is this secondary definition we are using when
discussing hybridization of dogs, or crossing of different breeds.

Let’s begin the discussion of hybrid vigor by talking about a couple
basic genetics principles.  Hang in there!  I’ll keep it short and simple.

Sets of genes control every characteristic of an organism. Each parent
contributes one gene, called an allele, so a set consists of 2 genes, or 2
paired alleles .  A dominant allele will cause that trait to show up even
if only one copy is present.  An allele that is recessive needs 2 copies in
order for the trait to show up.  For example, in humans, brown eyes
are dominant, and blue eyes are recessive.  If a person has an allele for
brown and an allele for blue, they will have brown eyes.  The dominant
brown overrides the recessive blue.  For a person to have blue eyes
they need to have TWO copies of the allele for blue eyes.  Notice in this
example that carrying the recessive trait does not influence the physical
result. The recessive gene’s presence is completely hidden.  

This is actually a helpful way to think of recessive traits: as hidden and
very sneaky.  A dog can carry such a trait and show no signs of it at
all.  But if bred to a dog of similar genetic composition, that is carrying
that hidden gene, some of the pups will end up with 2 copies of that
hidden gene.  With 2 copies of the hidden, defective trait gene, and NO
normal copies, the dog will have a genetic disorder that was not
evident in the parent.

A recessive trait, as you just learned, needs 2 copies in order for the
trait to show up. If a dog carrying one recessive (hidden) allele for
some genetic disorder is bred to a dog also carrying one recessive
(hidden) allele  for that disorder, some of the pups will get 2 copies
and show the disorder. Those genes have become more ‘concentrated’
in the population.  As one produces successive generations of a certain
breed, trying to concentrate and ‘fix’ the traits that define the breed,
other traits become concentrated and ‘fixed’ as well, those traits that
cause genetic disorders.  That’s why certain disorders are more
common in some breeds than others.  Nearly half of hereditary
diseases found in dogs occur predominantly or exclusively in one or
just a few breeds.  If a dog carrying the recessive, defective allele is
bred to a dog with ONLY normal versions of that allele, some pups will
be carriers, but none will show the disorder.  This is the basis of hybrid
vigor.

Hybrid vigor is the phrase commonly used for what is correctly called
heterosis.  That is,  the possibility that one may obtain a better
individual by combining the virtues of the parents, by preventing the
concentration of undesirable traits within the group.   Individuals that
are members of a population share genes, that’s what makes them
members of that population!  In the case of dogs, these different
populations are different breeds, and those genes define every
characteristic that  makes a dog a recognizable member of that breed.  
It takes differences in only 10 to 30 genes to define one breed from the
next.  So will crossing two breeds result in a healthier animal?  Maybe,  
it depends on whether the two breeds have in common any genetic
disorders, or defective alleles.

Expanding on that thought, consider what happens if we cross 2 breeds
who SHARE a genetic disorder.  For example, Hip Dysplasia is a
genetically based disorder controlled by a number of sets of genes, and
is  found in all foundation stock for doodles: Poodles, Golden
Retrievers, and Labrador Retrievers. So crossing these breeds would
NOT result in hybrid vigor for that particular disorder.  The defective,
recessive alleles could be contributed from any choice of parents,
resulting in pups that have multiple copies of the defective alleles, and
therefore show the disorder.

There are genetically-based disorders that are NOT found in all those
breeds.  An eye disorder called prcd-PRA is found in Mini and toy
poodles, and Labradors, but is very, very rare in Standard poodles and
Golden Retrievers.  Similarly, Lymphoma is found commonly in
Goldens and Poodles but not in Labs. Osteosarcoma is more common
in Labs, but not common in Goldens and Poodles.  Narcolepsy and  von
Willebrand’s Disease is found in Labs and Poodles, but not in Goldens.  
So hybrid vigor will occur for some disorders and not others,
depending on the type of doodle.  It is easy to see how we can avoid
concentrating the genes for certain disorders by crossing a breed that
carries a disorder with one that does not.

So hybrid vigor occurs for specific disorders, and it can also occur in
terms of general disease resistance.  There is a set of genes called the
major histocompatibility complex (MHC) consisting of thousands of
potential allelic combinations. The genes of the MHC are involved in
controlling disease resistance, immune function, and reproduction.  The
long term viability of any breed depends on maintaining a high degree
of genetic diversity in the MHC. The loss of MHC genetic diversity is
responsible for a portion of the reduced “hybrid vigor” in some breeds.  
These breeds are literally dying due to a lack of diversity in this
complex, and researchers are working on the development of canine
MHC genetic markers, so breeders can select and match these genes to
maximize health.

In the last couple decades, several breeds have been in danger of being
bred into oblivion due to the concentration of genes carrying genetic
disorders. Breed clubs responded to this by doing what to some was
absolutely heretical.  They outcrossed with unrelated breeds.  The AKC
saved the Dalmatian from extinction (a breed of deaf dogs was
undesirable, no matter what other characteristics they had!) by allowing
breeding to a Pointer, although they later rescinded the registration of
the progeny.  In Europe, Dutch Shepherd Dogs were outcrossed with
the Belgian Tervuren, and Bernese Mountain Dogs were crossed with
Newfoundlands. The choice was made to save these breeds by taking
advantage of the phenomenon of heterosis (hybrid vigor) to strengthen
them.

Hybrid vigor has been clearly shown to exist in everything from fruit
flies to orchids to pigs to humans.  That’s why there are laws against
people intermarrying , and why certain families that DID intermarry,  
like the Russian czars, find disorders like hemophilia among their
members.  It does not matter what you are breeding, by maximizing
the number of different alleles in the gene pool, you minimize the
chances that disease-causing genes will end up paired together in any
individual.   Therein lies the promise of heterosis, or hybrid vigor.

Author: Helene Roussi
Hybrid Vigor in Labradoodles and Goldendoodles
Westwood Doodles:
Labradoodles, Goldendoodles,
and North American Retrievers
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