The prevalence of genetic disease in dogs today can only be
described as alarmingly common which is bad news for dog owners and dog
lovers alike. The following is a list to illustrate the magnitude of
the problem:
1. On average all dogs carry at least 4-5 defective genes.
2. Over 500 genetic diseases have been identified in dogs.
3. Hip Dysplasia (HD) (an inherited orthopedic disease that may
result in the dog becoming lame) commonly occurs in 60 dog breeds and
occurs less frequently in another 110 breeds; thus this disease is seen
in over 170 breeds of dog.
4. 119 dog breeds are commonly afflicted with progressive
retinal atrophy (PRA), a disease that quite often leads to blindness in
the affected animal.
5. Of all the dog breeds the Poodle suffers from the most number
of genetic diseases...145! The reason why the Poodle is predisposed to
so many genetic diseases is because not only are there 3 distinct sizes
of Poodle in this breed, the large count of disease is a reflection of
the popularity of this dog breed. By and large a good rule of thumb is,
the more popular a dog breed the greater the number of genetic diseases
inherent in that breed.
6. Several popular dog breeds are linked to over 100 genetic diseases.
7. These days dog breeders spend well over $500,000,000 annually
in an effort to address this disturbing trend of genetic disease in
man's best friend.
As it is all the various dog breeds that exist today were artificially
created; in other words each and every dog breed was selectively bred
for a particular trait, be it coat color, coat length, sniffing
ability, shortness of stature (miniatures), facial appearance and so
forth...the list is virtually endless. The means by which those desired
traits were obtained was to selectively breed that population of dogs
that most strongly exhibited the desired trait; a breeding strategy
that is otherwise known as inbreeding!
Gene Pool And Population Factors
A population may be described as a breeding group that possesses gene
continuity from one generation to the next. Currently a growing body of
dog experts believe that the dog evolved as a new species from the wolf
to occupy a developing niche about 15,000 years ago. That developing
niche revolved around human waste dumps; opportunistic wolves began
inhabiting those waste dumps for easily available food supplies.
All told, 15,000 years on the evolutionary scale is an extremely brief
period for a new species to evolve from another, suggesting that there
must have been a considerable amount of inbreeding amongst those
opportunistic waste-dump-frequenting wolves to propagate the tameness
trait in so short a time span! Compounding this issue of limited
genetic pool, a growing number of dog researchers now believe that the
original genetic ancestry of the dog evolved from only three female
wolves that inhabited China several thousand years ago (the so called
eves of dog evolution).
The important point to note here is that even before mankind began his
intensive trait-specific breeding program of the dog, and due to its
unorthodox super-accelerated evolution, the dog gene pool right from
the get go was rather limited!
Development Of A Dog Breed
By its very nature the development of a new dog breed involves a
considerable amount of inbreeding to magnify and evolve the desired
trait(s) characteristic of that new breed. However this process results
in a considerable number of undesired results. If we consider the
Dalmatian, a breed of dog characterized by its distinctive pattern of
spots, somewhere along the line during that process of selecting for
the spotted pattern trait, Dalmatian dog breeders unknowingly and
unintentionally also bred this dog for an abnormal uric acid gene!
By the time people realized that there was a very serious problem in
the Dalmatian, this dog breed was homozygous for the abnormal uric acid
gene! (Homozygous refers to identical pairs of genes that manifest as
an observable trait as opposed to non-identical gene pairs known as
heterozygous that confer no observable trait; i.e., recessives). This
unwanted side effect means that Dalmatians are perhaps the only breed
of dog that is predisposed to urine stones (from excessive levels of
uric acid), a debilitating urinary tract condition. In an effort to
rectify this problem the Dalmatian line was subsequently crossbred with
various Pointer breeds to eliminate the inherent homozygotic uric acid
gene whilst still maintaining the spotted pattern that defines the
Dalmatian dog.
Gene Linkage
The abnormal elevated levels of uric acid in the Dalmatian associated
with the spotting-pattern gene, is a classic case of gene linkage.
Linkage is said to occur when genes expressing different properties are
located on the same chromosome and are inherited as a unit. In fact the
occurrence of gene linkage probably explains how the dog
morphologically (shape wise) differentiated from the wolf so
dramatically and so quickly; the gene that expressed the tameness trait
was linked to genes that expressed other properties such as floppy
ears, change in coat color, barking or simply put properties
characteristic of today's domesticated dog.
The bottom line here is that selective breeding for a particular trait
(as happens whenever a new dog breed is being developed) carries the
inherent risk of magnifying the expression of unwanted defective genes
that would otherwise rarely occur under natural circumstances.
Genetic Diversity
During the process of natural selection genetic diversity is
maintained, but contrastingly in selective breeding the opposite holds
true; genetic diversity is reduced! Recent studies have revealed that
genetic variation occurs frequently in normal populations of any
species and that even those genes that are deleterious and defective
are preserved in the gene pool as so-called recessive genes (recessive
gene traits are not visibly apparent unlike dominant genes). It is now
believed that such genetic variability confers adaptive properties to a
population in the event of extreme environmental change.
Thus mutative genes that confer minimal benefit today could play a
significant future role in the survival of the species in which it
occurs. This could explain why defective or mutative genes are
preserved as recessive genes in a population's gene pool. A good
example illustrating the beneficial role recessive genes can confer to
a population is the case of the English Peppered moth.
Like a large number of insects the Peppered moth relies on its color to
camouflage it from predators. The dominantly prevalent grayish color of
the Peppered moth allows it to blend in with its environment which so
happen to be tree trunks and lichen of like color. Although lighter and
darker shades of the moth exist, few of such individuals survive for
the simple reason their camouflaging abilities are less effective.
However with the advent of the industrial revolution in the late 19th
century the resultant pollution darkened the tree trunks such that the
minority recessive trait which conferred a darker shade of gray to the
moths became dominant because such moths were then better able to avoid
predation. As pollution levels subsequently decreased the tree trunks
gradually reverted to their normal color and so also did the color of
the Peppered moth revert back to its pre-industrialization gray. Thus
once again the recessive dark-gray color in the moth reverted to its
usual minority role!
Combating Dog Genetic Disease
Because almost every single dog possesses at least 4 defective genes
(no matter whether it's a purebred or mixed breed) unless strong active
measures are taken the problem of dog genetic disease can only get
worse. Such strong effective measures by necessity require the
participation of everyday dog owners and not just dog breeders or dog
researchers!
Dog Genome Project Wants Your Dog's Blood!
The Canine Genome Sequencing Project (Dog Genome Project based at the
Broad Institute of Harvard and MIT in Boston) has sequenced the dog
genome in its entirety. One of their priority goals now is to identify
the genes responsible for causing dog genetic diseases, defective
genes, carrier states of hereditary disease and much more. Through
using the powerful resource of the sequenced dog genome, the good folks
at the Dog Genome Project hope to reverse the widespread prevalence of
genetic disease in dogs worldwide.
However to achieve that noble goal they need your help! How?
Well the only means by which recessive genes and carrier states can be
identified is at the genetic level (genotype) and not by visual
observation of recognizable morphological traits (phenotype). To do
this the Dog Genome Project staff need a large number of dog samples to
analyze and that is where you, as a dog owner, can play a significant
role by sending them a blood sample of your dog.
For more information about how to help alleviate dog genetic disease go to the following url:
http://www.broad.mit.edu/mammals/dog/pdfs/dog_project_info.pdf
Dog Breeds
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