The Downside of Inbreeding
The Downside of Inbreeding
IT’S TIME FOR A NEW APPROACH
"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
behaviours 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, vigour
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, colour or coat-type lines, with dogs of one
colour 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 behaviours, such as herding
ability, can be diminished in this manner, if breeders are not using the
behaviour 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.
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.
Copyright
1999 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.
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