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In my travels to the shows since I became a
Canadian Kennel Club Director, I have had a lot of time to talk
to people in many breeds, and their main concerns are the health
problems, genetic defects and longevity of their breeds. In some
breeds these problems seem to have reached epidemic proportions.
It was reported in a
letter in English Dog World that in a study carried out in
Cavalier King Charles Spaniels, 57% were found to be suffering
from heart murmurs by the age of five years. In Sweden the
problem is even worse, and a study there concluded that this
problem probably originated from one or more of the foundation
dogs in the 1920s. Many breeds have a multitude of problems, and
it appears that some of the most affected breeds are those which
arose from a very limited number of foundation animals.
I have been involved
with Salukis for 35 years, and in that time I have seen them go
from a breed with virtually no health problems and a 15-year
life span to a breed suffering from frequent early sudden death
from various causes (common enough to have been labelled Saluki
Sudden Death Syndrome with a research fund set up) and many
other health problems such as thrombocytopenia, thyroid
abnormalities and allergies.
The saluki, although a
very ancient breed that spanned a wide geographical area, went
through a genetic bottleneck and exists in our "purebred
registered" world as a result of a handful of imports from the
early 1900s and later, and although our four or five generation
pedigrees may indicate that dogs may be not closely related,
they are in fact descended from the same few individuals, and
thus have a small gene pool. In the U.S.A. there have been
numerous new imports from Arabia, not registerable of course,
since they were not registered in their country of origin, but
their fanciers have breed them and there are now some 200
unregistered Salukis of new bloodlines. So far, I have been
told, these dogs show vigorous good health, outstanding hunting
ability and longevity.
I have recently read
Last Animals At The Zoo - How Mass Extinction Can Be Stopped by
Colin Tudge (from my local public library) and the chapter on
the theory of conservation breeding gives food for thought to
breeders of purebred dogs, because with the small numbers of
foundation animals in most breeds they are genetically analogous
to small populations of endangered species.
Briefly, the most important principles in
conservation breeding are first -- at the level of the
individual -- that it is important to avoid inbreeding
(including what we call linebreeding), brought about by mating
of animals who are closely related to each other. Second -- at
the level of the population -- it is vital for long term success
to conserve genetic variation.
In humans it is
estimated that one in 20 North European people -- an
extraordinarily high proportion -- carry the recessive gene that
causes cystic fibrosis, when inherited in double dose. Since
people do not usually mate with relatives, each carrier has a
one in 20 chance of marrying another carrier, so the chances of
two carriers marrying are one in 400. One in four children of
such a marriage will have the disease. So we would expect to
find that one in 1,600 North European infants is born with
cystic fibrosis, which is precisely what we do find. If a
recessive disease gene were carried by one in 50 people (which
we would consider common enough) then the disease would appear
only once in 10,000 infants. It is estimated that each human
being carries an average of about six recessive deleterious
alleles which would cause serious disease. Thus we maintain the
health of the population by outcrossing, which minimizes the
chance of deleterious genes doubling up. We do not try to weed
out all the carriers of all the diseases from the breeding
population!
In dogs, we know how to test for specific
problems such as hip dysplasia and eye abnormalities, but there
are many other factors in a dog's genetic makeup which are
unknown, and which may only come to light several generations
down the line. At the same time, we are selecting for other
factors such as working ability, temperament, and, of course,
show ring success. Show ring success has traditionally been
easier to achieve by line-breeding and inbreeding, which
accounts for its popularity with show breeders, and by extensive
use of the top animals, especially stud dogs. According to
geneticists, the effective population of a group of animals
cannot be more than four times the number of different sires.
Thus if four stud dogs are breed to a total of 100 bitches,
resulting in 500 puppies, the effective population is only 16.
See how easy it is to dramatically reduce the gene pool of a
breed? In a numerically small breed, the popularity and
extensive use of a few stud dogs can produce a genetic
bottleneck.
It is impossible to test for every deleterious allele a dog may
carry, and it can be assumed that every dog carries some,
therefore several generations down the line we may find, when we
start breeding a popular stud dog's descendants to each other,
that we have a genetic problem which has now been passed on to
hundreds of individuals.
Overuse of a few dogs can also greatly reduce genetic variation,
because of genetic drift. For example, take these 100 litters,
and assume that the breeder in each case keeps the pick of
litter for further breeding. Each of these puppies has received
half its genes from its sire, and half from its dam. Thus, the
other half of the dam's genes, which she did not pass on to this
puppy, are lost forever if that is her only offspring that is
bred. To retain as many different genes as possible in a breed,
it is necessary to breed from as many different animals as
possible.
In Europe some breed clubs have imposed a
limit on the number of litters a stud dog may sire in his
lifetime, in order to prevent the reduction of the gene pool and
ensure the genetic diversity and health of the breed. Europe is
fortunate to have geneticists who specialize in the study of
dogs, and who have kept up with the advances in population
genetics, and who share their knowledge with breeders. The breed
club here could appoint committees to search out this knowledge
to share with its members, and should be prepared to make
recommendations to breeders, enforceable through the code of
ethics, to prevent overuse of stud dogs and other breeding
practices which could cause long-term harm to the breed.
Owners of stud dogs could also assume
this responsibility and, in the best interests of the breed,
limit the number of litters a dog would sire. I believe that
this is the most important issue facing breeders today, and I
hope you will think seriously about your breeding programs and
the future health and welfare of your breed.
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