Article Of Interest
The Canine Diversity Project
The Price of Popularity: Popular Sires and Population Genetics
C.A. Sharp
Consider the hypothetical case of Old Blue,
Malthound extraordinaire. Blue was
perfect: Sound, healthy and smart. On week days he
retrieved malt balls from dawn to
dusk. On weekends he sparkled in malt field and
obedience trials as well as
conformation shows, where he baited to--you guessed
it--malt balls.
Everybody had a good reason to breed to Blue, so
everybody did. His descendants
trotted in his paw-prints on down through their
generations. Blue died full of years and
full of honor. But what people didn't know was that Old
Blue, good as he was, carried
a few bad genes. They didn't affect him, nor the vast
majority of his immediate
descendants. To complicate the matter further, some of
those bad genes were linked to
genes for important Malthound traits.
A few Malthounds with problems started showing up.
They seemed isolated, so
everyone assumed it was "just one of those
things." A few declared them "no big deal."
Those individuals usually had affected dogs. All in
all, folks carried on as usual.
Time passed. More problem dogs turned up. People
made a point not to mention the
problems to others because everyone knows the stud
owner always blames the bitch
for the bad tings and takes credit for the good. Stud
owners knew it best to keep quiet
so as not to borrow trouble. Overall, nobody did
anything to get to the bottom of the
problems, because if they were really significant,
everybody would be talking about it,
right?
Years passed. Old Blue had long since moldered in
his grave. By now, everyone was
having problems, from big ones like cataracts, epilepsy
or thyroid disease to less
specific things like poor-keepers, lack of mothering
ability and short life-span. "Where
can I go to get away from this?" breeders
wondered. The answer was nowhere.
People became angry. "The responsible parties
should be punished!" Breeders who felt
their programs might be implicated stonewalled. Some
quietly decided to shoot, shovel
and shut-up. A few brave souls stood up and admitted
their dogs had a problem and
were hounded out of the breed.
The war raged on, with owners, breeders and rescue
workers flinging accusations at
each other. Meanwhile everybody carried on as always.
After another decade or two
the entire Malthound breed collapsed under the weight
of its accumulated genetic
debris and went extinct.
This drastic little fable is an exaggeration--but
not much of one. Here's similar, though
less drastic, example from real life: There once was a
Quarter Horse stallion named
Impressive. The name fit. He sired many foals who also
exhibited his desired traits. But
when they and their descendants were bred to each
other, those offspring sometimes
died. Impressive had been the carrier of a lethal
single-gene recessive trait. No one
knew it was there until they started in-breeding on
him. The situation of a single sire
having this kind of drastic genetic effect on a breed
became known as the "Impressive
Syndrome."
Many species and breeds of domestic animals,
including dogs, have suffered
"Impressive Syndromes" of their own. But
cases like that of Impressive are only the tip
of the iceberg. A single-gene recessive becomes obvious
in just a few generations. But
what about more complex traits?
This is not to say that those popular sires we so
admire are bad breeding prospects.
Their many excellent traits should be utilized, but
even the best of them has genes for
negative traits.
The problem is not the popular sires, but how we use
them. For a century or more,
in-breeding has been the name of the game. (For the
purposes of this article,
"in-breeding" refers to the breeding of dogs
related to each other and therefore includes
line-breeding.) By breeding related individuals, a
breeder increased his odds of
producing dogs homozygous for the traits he wanted.
Homozygous individuals are
much more likely to produce those traits in the next
generation.
When a male exhibits a number of positive traits and
then proves his ability to produce
those traits he may become a popular sire, one that is
used by almost everyone
breeding during his lifetime, and maybe beyond, thanks
to frozen semen.
Since the offspring and grand-offspring and so on
are good, breeders start breeding
them to each other. If the results continue to be good,
additional back-crosses may be
made for generations. Sometimes a sire will be so
heavily used that, decades hence,
breeders may not even be aware of how closely bred
their animals are because the dog
no longer appears on their pedigrees.
This is the case in Australian Shepherds. Most
show-line Aussies trace back,
repeatedly, to one or both of two full brothers:
Wildhagen's Dutchman of Flintridge
and Fieldmaster of Flintridge. These, products of a
program of inbreeding, were
quality individuals and top-producing sires. They are
largely responsible for the over-all
quality and uniformity we see in the breed ring
today--a uniformity that did not exist
before their birth nearly three decades ago.
Working lines have also seen prominent sires, but
performance traits are far more
complex, genetically and because of the significant
impact of environment. They are
therefore harder to fix. Performance breeders will
in-breed, but are more likely to
stress behavioral traits and general soundness than
pedigree and conformational
minutiae. The best working sires rarely become as
ubiquitous as the best show-line
sires.
Not every popular sire becomes so because of his
ability to produce quality offspring.
Some have won major events or are owned by individuals
with a knack for promotion.
Such dogs may prove to be wash-outs once their get is
old enough to evaluate. But a
lot of breeders have been using the animal for the few
years it takes to figure that out,
the damage may already have been done.
Use of even the best popular sires, by its very
nature, limits the frequency of some
genes in the breed gene pool while simultaneously
increasing the frequency of others.
Since sons and grandsons of popular sires tend to
become popular sires the trend
continues, resulting in further decrease and even
extinction of some genes while others
become homozygous throughout the breed. Some of these
traits will be positive, but
not all of them.
The owners of Old Blue, the Malthound in the opening
fable, and those who owned his
most immediate descendants had no idea what was
happening under their noses. They
were delighted to have superior studs and even more
delighted to breed them to as
many good bitches as possible.
Dog breeding and promoting is an expensive
proposition. One usually winds up in the
hole. But owning a popular sire can change that. The
situation looks like a winner for
everyone--the stud owner finds his financial burden
reduced while breeders far and
wide get to partake of his dog's golden genes.
No one breeding dogs wants to produce sick dogs. A
small minority are callous and
short-sighted enough to shrug genetic problems off as
the price you pay to get winners,
but even they do their best to avoid letting it come to
general attention.
We need a total re-thinking of how we utilize stud
animals. No single dog, no matter
how superior, should dominate the gene pool of its
breed. Owners of such sires should
give serious consideration to limiting how often that
dog is used, annually, through its
lifetime and on into the future, if frozen semen is
stored. The stud owner should also
look not only at the quality of the bitches being
presented, but their pedigrees. How
much will the level of inbreeding be increased by a
particular mating?
The bitch owner also needs to think twice about
popular sires. If you breed to the stud
of the moment and everyone else is doing the same,
where will you go when it comes
time to make an outcross?
Finally, the attitude toward genetic disease itself
has to change. It must cease being
everyone's dirty little secret. It must cease being a
brick with which we bludgeon those
with the honesty to admit it happened to them. It must
become a topic of open,
reasoned discussion so owner of stud and bitch alike
can make informed breeding
decisions. Unless breeders and owners re-think their
long-term goals and how they
react to hereditary problems, the situation will only
get worse.
C.A. Sharp is editor of the "Double Helix
Network News". This article appeared in
Vol. IV, No. 3 (Summer 1998). It may be reprinted
providing it is not altered and
appropriate credit is given.
August 18, 1998
The Downside of Inbreeding: It’s
Time For a New Approach
C.A. Sharp
"Inbreeding was
once a valuable tool in shaping today’s breeds. As these
have now reached a high
degree of homogeneity, it has lost its importance
and turned into a fatal
and disastrous habit."
Hellmuth Wachtel, PhD
Inbreeding (which, for the purposes of this article,
includes "linebreeding") has been the
rule in dog breeding for the better part of two
centuries. Before that, breeders bred
"like-to-like." Records may or may not have
been kept, depending on the literacy,
social status or interest of the breeder. Pedigrees
were of marginal interest, if they were
considered at all. Registries, as we know them now, did
not exist. New individuals
might be introduced to the breeding pool at any time,
so long as they displayed
characteristics that the breeder wanted to perpetuate.
Even an unplanned mating with a
dog that would never have been deliberately selected
might be shrugged off so long as
some of the offspring proved useful.
In the nineteenth century, prominent European
breeders of various domestic species,
including dogs, became interested in maintaining the
"purity" of their bloodlines. They
had no knowledge of genetics, indeed the science had
yet to be born. Their breeding
theories were a reflection of social attitudes of the
times. It should also be kept in mind
that these individuals were mostly wealthy men whose
human pedigrees were
considered better than those of "common"
people. As pedigrees became more
important, so did the regular appearance of significant
names in those pedigrees.
Eventually registries were established to keep official
records. At some point, virtually
all dog registries became closed. Most of this occurred
before breeders had even a
rudimentary knowledge of genetic science.
At first, inbreeding proved beneficial. Breeders
learned that by mating related
individuals of the desired type, the resulting quality
and uniformity of the offspring
improved As people began to learn basic genetics in the
early part of this century, they
deliberately sought to fix desired traits, particularly
in production livestock, by breeding
near relatives. This practice continues to the present
day. A sire will be
"progeny-tested" by being bred to a group of
his daughters. If the offspring measure up,
he will be kept for stud. If they don’t, everybody
goes to market. This drastic culling
serves its purpose in livestock, but it is impractical
and unacceptable in companion
animals such as dogs.
Nature goes to great lengths to discourage
inbreeding. Related animals rarely mate,
which prevents genes for diseases and defects from
coming together with any great
frequency. Wild animals have a variety of behaviors
which will eliminate or severely
restrict inbreeding. In wolves, the species most
closely related to dogs, only the alpha
pair will breed. Pups stay with the pack for their
first year. After that time they must
find a place, often low-ranking, within the adult
hierarchy. If a yearling cannot accept
this or it becomes the brunt of too much negative
social interaction, it will disperse.
Dispersers may have to travel many miles before they
can find an available territory and
a mate, if they can find them at all. Those individuals
which do not disperse will not be
breeders unless they should someday attain alpha
status, so the breeding of relatives is
unlikely.
Sometimes circumstances give animals no choice but
to mate with relatives. If those
conditions persist for any length of time they create a
"genetic bottleneck." The wolves
of Isle Royale in Lake Michigan descend from a very
small number of animals which
crossed from the mainland decades ago during a hard
winter when the lake froze over.
Their present-day descendants have proved more than
usually vulnerable to an
assortment of diseases and parasites. When canine
parvovirus reached Isle Royale, the
wolf population plummeted so badly that some observers
at the time feared the wolves
would die out entirely.
In recent years, purebred dogs have experienced
increasing problems with hereditary
diseases and defects. The causes are complex, including
genetic load, the presence of
lethal equivalents in all individuals, genetic
bottlenecks, closed gene pools, gene pool
fragmentation, and genetic drift, but all are
attributable to inbreeding.
Thanks to closed registries, breeds form exclusive
gene pools. All gene pools, no
matter how large or diverse, will have a genetic load
— the difference between the
fittest possible genotype and the average fitness of
the population. "Fitness" is the
individual’s over-all health, vigor and ability. It
may or may not directly relate to traits
breeders select for. (The English Bulldog, for
instance, has an "ideal" physical form
which virtually precludes females from being able to
naturally whelp their young.) The
greater the genetic load, the more genetic difficulties
members of a breed are likely to
suffer. In a closed gene pool, the situation may remain
stable or deteriorate. It cannot
get better.
Each individual within a breed also carries it’s
own kind of load — four or five genes
for potentially fatal diseases or defects. These are
called "lethal equivalents." In most
cases they will not affect the individual carrying them
because a single allele, or form of
the gene, will be insufficient to cause the problem.
But when relatives are mated, the
odds of matching up those alleles increases and as does
the frequency the disease.
Every population must deal with genetic load and
lethal equivalents, but when the
population is prevented having genetic exchange with
other similar populations, genetic
diversity within the population begins to diminish.
Some of this may be beyond
anyone’s control. A breed’s function may have
become obsolete, resulting in only a
few surviving members. This was the case with the
Portuguese Water Dog. All
present-day PWDs descend from a handful of dogs.
Social, political or environmental
difficulties may also preclude breeding, causing
populations to crash. Many breeds
experienced a genetic bottleneck at the time of World
War II. With much of the world
at war, dog breeding was not a high priority and
populations in areas of military action
were often wiped out or severely depleted. In such a
situation, breeders can only make
do with what remains. It’s a tough row to hoe for the
truly rare breeds, especially since
the prevailing attitude that breeds must be kept
"pure" prevents supplementing with
fresh genetic material from similar, less impacted,
populations.
Breed gene pools can fragmented into so many gene
puddles when they are arbitrarily
split along size, color or coat-type lines, with dogs
of one color or variety prohibited
from mating with those of another. No matter how
diverse a breed may have been
before such distinctions were made, afterwards breeders
have fewer options when
choosing mates and the eventual result will be
increased inbreeding because there isn’t
anywhere else to go. One striking example of this is
the Belgian Sheepdog in the United
States. Outside the US this breed contains four
varieties, all of which might occur in a
single litter. The American Kennel Club lists three of
varieties as entirely separate
breeds. The fourth isn’t even recognized. In the US
they cannot be interbred though
throughout the rest of the world, they can.
Changes in social conditions may also fragment breed
gene pools. The Australian
Shepherd was originally a working ranch and farm dog.
Today there are far more
Aussies than there are "jobs" on farms and
ranches; so most are companion animals.
Over the past three decades, the breed has clearly
split between working and
conformation strains with a third, smaller, category of
"versatility" animals whose
breeders work toward a multi-purpose animal .There is
also a population of "mini"
Aussies—dogs whose size is below the breed norm. They
are often registered as
Australian Shepherds along with listing in a registry
for minis. There is very little
breeding between these various sub-groups though all
trace back to more-or-less
overlapping sets of founder animals.
One of the results of gene pool fragmentation is
loss of alleles that may exist in the
breed but didn’t happen to occur in the founders for
that variety. Genetic drift can
cause further loss. Genes not being specifically
selected for tend to "drift" out of the
gene pool. Many of these will be for things so subtle
they might never come to a
breeder’s direct attention. A dog has some 100,000
genes, only a relative few of which
are for things we can readily observe or measure. Many
of these genes cause minor
variations in form or bodily function. Cumulative
losses of such genes through genetic
drift can reduce overall health and fitness without
presenting consistent or identifiable
signs; a dog may seem to be a poor keeper, unusually
subject to minor ailments, or
lacking in endurance. Even "typical" breed
behaviors, such as herding ability, can be
diminished in this manner, if breeders are not using
the behavior as part of their
selection criteria.
The use of popular sires, particularly multiple
generations of them, can accelerate loss
of alleles. A dog can only have a maximum of two
alleles for any given gene. Excessive
use of a single individual will skew the gene pool
toward the alleles that dog happened
to carry. Obviously, such a dog gets heavy use because
he has desirable traits. Genes
for those traits will become more common, but so will
those for his lethal equivalents
and more subtle ills. And if a deleterious gene is
"linked" (sits close on the
chromosome) to a desired gene the sire carries, the
breed may suddenly find itself
riddled with the problem that bad gene causes. It won’t
be easy to eliminate unless
breeders are also willing to give up the linked desired
trait.
Proponents of inbreeding often point out that
mongrels have more genetic problems
than purebreds. While it is true that mongrels, as a
group, have more individual kinds of
diseases and defects than any single pure breed, it
must be remembered that each
breed represents only a portion of the canine gene
pool, whereas mongrels encompass
all of it. If mongrels’ defects are compared to those
found among all pure breeds, the
discrepancy disappears. Since mongrels usually are the
result of random, unplanned
breeding, the incidence of defects is low in the
overall population. In pure breeds many
of those same defects are common. For instance,
progressive retinal atrophy and collie
eye anomaly are rare in mongrels. Incidence of both is
high in Collies.
It is becoming more and more apparent that the
short-term gains of inbreeding are
outweighed by its long-term costs. Present-day breeders
need to re-think their
strategy. Assortative mating—the mating of
phenotypically similar but unrelated or
less-related individuals—will allow breeders to reach
their breeding goals while
reducing the loss of alleles in the over-all
population. To accomplish this it is vital that
each breeder has a thorough knowledge of breed
pedigrees. The typical three to five
generation pedigree may indicate few, if any, common
ancestors. But what happens if
the pedigree is extended a few more generations? If two
dogs share no ancestors for
four generations, but share many in the 5th, 6th and so
on, breeding them would be
inbreeding.
All members of a single breed are, of course,
related to some degree, though how
much varies from breed to breed. Somewhere back in each
breed’s history there is a
group of founders from whom all present-day dogs
descend. Portuguese Water Dogs
have very few, Australian Shepherds have quite a
number, though not every Aussie
goes back to all of them. It is important to know who
the founder individuals were,
particularly if the breed is rare, split into varieties
or experienced a significant bottleneck
at some point in its history. A large number of
founders allows for greater diversity
(assuming those founders were, themselves, unrelated),
but if some are heavily
represented in comparison to others due to inbreeding
on their descendents, diversity is
at risk. Breeders should strive to increase the
representation of the neglected founders
whenever possible.
Calculation of inbreeding coefficients will give an
indication of how inbred a dog or a
prospective cross is. Knowing these numbers enables the
breeder to make choices that
will reduce inbreeding. Good books on animal breeding
will have a section explaining
how this is done, but calculating them by hand becomes
cumbersome when working
with a full pedigree. There are pedigree programs on
the market which will perform
these calculations.
Perhaps the most important issue is making health a
top priority. It is obvious even to
those who promote inbreeding that screening for genetic
diseases and not breeding
affected individuals is important. As tests become
available which will detect carriers of
genetic problems, they should be put to use. However,
carrier status should not
automatically preclude breeding of otherwise good
individuals. Care should be taken
that they aren’t bred to other carriers and those who
buy puppies from a carrier parent
should be advised to screen the pup if they want to
breed it. But eliminating proven
carriers as breeding stock is throwing our their many
good genes while avoiding one
bad one.
Australian Shepherd breeders are doing this with
Pelger-Huet Anomaly. PHA is lethal
to offspring that inherit two copies of the gene,
resulting in reduced litter size and
neonatal deaths. Carriers rarely suffer any effects.
Knowledgeable breeders use a
blood test to screen and carriers are bred to
non-carriers.
Less specific aspects of health must also be
considered. A dog that is a "hard keeper,
or repeatedly comes down with one minor ill or another
should not be a breeding
prospect. These individuals likely carry a surplus of
genes which individually have only
a small negative effect on health but cumulatively have
produced an unthrifty individual.
A common result of inbreeding is "inbreeding
depression," typified by small litter size or
difficulty producing or rearing young. Bitches from
families that consistently produce
small litters may be suffering inbreeding depression.
Animals which can only be bred or
raise their puppies if they receive extraordinary human
assistance are poor breeding
candidates. This is not to say that people shouldn’t
properly house and care for their
animals, but if a dog is indifferent to bitches in
standing heat or a bitch needs to be
physically restrained to keep her from resorting to
fight or flight in an attempt to prevent
mating, or won’t settle without veterinary
intervention, or is apt to kill or damage her
puppies through intent or neglect, these are signs of
inbreeding depression and that
animal shouldn’t be bred. Breeders should not go to
excessive, near surgical, lengths to
control the environment for newborns, nor should they
use heroic measures to keep
failing whelps alive. (For those who find this too
callous: Save them if you will, but
don’t breed them.)
Inbreeding gave us the many breeds of dog we enjoy
today, but its time is past. If
purebred dogs are to remain viable into the next
century breeders need to rethink their
strategy and work toward their goals with more emphasis
on over-all health and
concerted efforts to reduce the level of inbreeding in
their dogs.
C.A. Sharp is editor of the "Double Helix
Network News". This article appeared in
Vol. VII, No. 1 (Winter 1999). It may be reprinted
providing it is not altered and
appropriate credit is given.
Feb. 26, 1999