PROS AND CONS OF INBREEDING
Copyright 1996, Sarah Hartwell
Adapted, with permission, from Cat
Recourse Archive and
edited by Dog Breed Info.
Inbreeding is the mating together of closely related dogs, for
example mother/son, father/daughter and sibling/sibling matings. For
breeders, it is a useful way of fixing traits in a breed - the pedigrees
of some exhibition dogs show that many of their forebears are closely
related. For example, there is a famous cat by the name of Fan Tee Cee (shown in the 1960s and
1970s) appeared in more and more Siamese pedigrees, sometimes several
times in a single pedigree, as breeders were anxious to make their lines
more typey. Superb specimens are always much sought after for stud
services or offspring (unless they have already been neutered!) having
won the approval of show judges.
However, inbreeding holds potential problems. The limited genepool
caused by continued inbreeding means that deleterious genes become
widespread and the breed loses vigor. Laboratory animal suppliers
depend on this to create uniform strains of animal which are immuno-depressed
or breed true for a particular disorder e.g. epilepsy. Such animals are
so inbred as to be genetically identical (clones!), a situation normally
only seen in identical twins. Similarly, a controlled amount of
inbreeding can be used to fix desirable traits in farm livestock e.g.
milk yield, lean/fat ratios, rate of growth etc.
NATURAL OCCURRENCE OF INBREEDING
This is not to say that inbreeding does not occur naturally. A
wolf pack, which is isolated from other wolf packs, by geographical or other
factors, can become very inbred. The effect of any deleterious genes
becomes noticeable in later generations as the majority of the offspring
inherit these genes. Scientists have discovered that wolfs, even if
living in different areas, are genetically very similar. Possibly the desolation
of their natural habitat has drastically reduced wolf numbers in the
past crating a genetic bottleneck.
In the wolf, the lack of genetic diversity makes them
susceptible to disease since they lack the ability to resist certain
viruses. Extreme inbreeding affects their reproductive success with
small litter sizes and high mortality rates. Some scientists hope that
they can developed a more varied gene pool by introducing wolfs from
other areas into the inbred wolf packs.
Another animal suffering from the effects of inbreeding is the
Giant Panda. As with the wolf, this has led to poor fertility among
Pandas and high infant mortality rates. As Panda populations become more
isolated from one another (due to humans blocking the routes which
Pandas once used to move from one area to another), Pandas have greater
difficulty in finding a mate with a different mix of genes and breed
less successfully.
In cats natural isolation and inbreeding have given rise to domestic
breeds such as the Manx which developed on an island so that the gene
for taillessness became widespread despite the problems associated with
it. Apart from the odd cat jumping ship on the Isle of Man, there was
little outcrossing and the effect of inbreeding is reflected in
smaller-than-average litter sizes (geneticists believe that more Manx
kittens than previously thought are reabsorbed due to genetic
abnormality), stillbirths and spinal abnormalities which diligent
breeders have worked so hard to eliminate.
Some feral colonies become highly inbred due to being isolated
from other cats (e.g. on a remote farm) or because other potential mates
in the area have been neutered, removing them from the gene pool. Most
cat workers dealing with ferals have encountered some of the effects of
inbreeding. Within such colonies there may be a higher than average occurrence
of certain traits. Some are not serious e.g. a predominance of
calico pattern cats. Other inherited traits which can be found in
greater than average numbers in inbred colonies include polydactyly (the
most extreme case reported so far being an American cat with 9 toes on
each foot), dwarfism (although dwarf female cats can have problems when
try to deliver kittens due to the kittens' head size), other structural
deformities or a predisposition to certain inheritable conditions.
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The ultimate result of continued inbreeding is terminal lack of vigor
and probable extinction as the gene pool contracts, fertility
decreases, abnormalities increase and mortality rates rise.
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SELECTIVE BREEDING
Artificial isolation (selective breeding) produces a similar
effect. When creating a new breed from an attractive mutation, the gene
pool is initially necessarily small with frequent matings between
related dogs. Some breeds which resulted from spontaneous mutation have
been fraught with problems such as the Bulldog. Problems such as hip dysplasia
and achalasia in the German Shepherd and patella luxation are more
common in certain breeds and breeding lines than in others, suggesting
that past inbreeding has distributed the faulty genes. Selecting
suitable outcrosses can reintroduce healthy genes, which might otherwise
be lost, without adversely affecting type.
Zoos engaged in captive breeding programs are aware of this need
to outcross their own stock to animals from other collections. Captive
populations are at risk from inbreeding since relatively few mates are
available to the animals, hence zoos must borrow animals from each other
in order to maintain the genetic diversity of offspring.
Inbreeding holds problems for anyone involved in animal husbandry
- from canary fanciers to farmers. Attempts to change the appearance of
the Pug in attempts to have a flatter face and a rounder head resulted
in more c-sections being required and other congenital problems. Some of
these breeds are loosing there natural ability to give birth without
human assistance.
In the dog world, a number of breeds now exhibit hereditary faults
due to the over-use of a particularly "typey" stud which was
later found to carry a gene detrimental to health. By the time the
problems came to light they had already become widespread as the stud
had been extensively used to "improve" the breed. In the past
some breeds were crossed with dogs from different breeds in order to
improve type, but nowadays the emphasis is on preserving breed purity
and avoiding mongrels.
Those involved with minority breeds (rare breeds) of livestock
face a dilemma as they try to balance purity against the risk of genetic
conformity. Enthusiasts preserve minority breeds because their genes may
prove useful to farmers in the future, but at the same time the low
numbers of the breed involved means that it runs the risk of becoming
unhealthily inbred. When trying to bring a breed back from the point of
extinction, the introduction of "new blood" through crossing
with an unrelated breed is usually a last resort because it can change
the very character of the breed being preserved. In livestock, successive generations of progeny must be
bred back to a purebred ancestor for 6 - 8 generations before the
offspring can be considered purebred themselves.
In the dog fancy, breed purity is equally desirable, but can be
taken to ridiculous lengths. Some fancies will not recognize "hybrid" breeds such as the
white or Parti-Schnauzer because it produces
variants. Breeds
which cannot produce some degree of variability among their offspring
risk finding themselves in the same predicament as wolfs and Giant
Pandas. Such fancies have lost sight of the fact that they are
registering "pedigree" dogs, not "pure-bred" dogs,
especially since they may recognize breeds which require occasional
outcrossing to maintain type!
IMPLICATIONS OF INBREEDING FOR THE DOG BREEDER
Most dog breeders are well aware of potential pitfalls associated
with inbreeding although it is tempting for a novice to continue to use
one or two closely related lines in order to preserve or improve type.
Breeding to an unrelated line of the same breed (where possible) or
outcrossing to another breed (where permissible) can ensure vigor.
Despite the risk of importing a few undesirable traits which may take a
while to breed out, outcrossing can prevent a breed from stagnating by
introducing fresh genes into the gene pool. It is important to outcross
to a variety of different dogs, considered to be genetically
"sound" (do any of their previous offspring exhibit
undesirable traits?) and preferably not closely related to each other.
How can you tell if a breed or line is becoming too closely
inbred? One sign is that of reduced fertility in either males or
females. Male dogs are known to have a low fertility rate. Small
litter sizes and high puppy mortality on a regular basis indicates that
the dogs may be becoming too closely related. The loss of a large
proportion of dogs to one disease indicates that the dogs are losing/have lost immune system diversity. If 50% of individuals
in a breeding program die of a simple infection, there is cause for
concern.
Highly inbred dogs also display abnormalities on a regular basis
as "bad" genes become more widespread. These abnormalities can
be simple undesirable characteristics such as misaligned jaws (poor
bite) or more serious deformities. Sometimes a fault can be traced to a
single male or female which should be removed from the breeding program
even if it does exhibit exceptional type. If its previous progeny are
already breeding it's tempting to think "Pandora's Box is already
open and the damage done so I'll turn a blind eye". Ignoring the
fault and continuing to breed from the dog will cause the faulty genes
to become even more widespread in the breed, causing problems later on
if its descendants are bred together.
In cats, one breed which was almost lost because of inbreeding is the
American Bobtail. Inexperienced breeders tried to produce a colorpoint
bobtailed cat with white boots and white blaze and which bred true for
type and color, but only succeeded in producing unhealthy inbred cats
with poor temperaments. A later breeder had to outcross the small
fine-boned cats she took on, at the same time abandoning the rules
governing color and pattern, in order to reproduce the large, robust
cats required by the standard and get the breed on a sound genetic
footing.
CONCLUSION
Inbreeding is a two-edged sword. On the one hand a certain amount
of inbreeding can fix and improve type to produce excellent quality
animals. On the other hand, excessive inbreeding can limit the gene pool
so that the breed loses vigor. Breeds in the early stages of
development are most vulnerable as numbers are small and the dogs may be
closely related to one another. It is up to the responsible breeder to
balance inbreeding against crossings with unrelated dogs in order to
maintain the overall health of the line or breed concerned.
