When trying to
eliminate genetic defects/disorders in canines, breeders must try to
identify the genetic status of their breeding stock. Being able
to verify the presence or absence of defective genes in a breeding dog
is a breeder's most powerful tool in ensuring the health of future
generations. The
inheritance pattern of many canine disorders has not been established.
Some inheritance patterns are known, or widely suspected, but others
are completely unknown.
When the inheritance pattern of a
specific gene is known or suspected, it can help breeders
to eliminate or minimize the appearance of that particular disorder.
When the inheritance pattern of a defective gene is unknown, breeders
have a much harder time eliminating that defect. In some
instances, the mode of inheritance of a disorder/defect may be
unknown, but members of certain breeds may be known to have an increased risk for
that disorder/defect when compared to dogs of other breeds. If a
breed shows a significantly increased risk for a certain
disorder/defect, that breed is said to have a
"predisposition" for
that condition.
When discussing
the inheritance pattern of genes, it must be understood that genes are
inherited in pairs. Every dog will have two (2) of each gene.
The dog will inherit one gene from each parent for each pair of genes.
In a very simplified explanation of gene inheritance, some of these
inherited genes will be dominant (expressed/seen with only 1 copy of
the gene) and some will
be recessive (hidden/unseen unless there are 2 copies of the gene).
The following
definitions are for known inheritance patterns:
Trait requires
only 1 copy of the defective gene, inherited from either parent, to
produce an "affected" dog (one that exhibits the defect). This dominant trait will be passed from the
affected parent to approximately half of its offspring. All
offspring that inherit this gene will be "affected". This type
of defect is relatively easy to eliminate so long as the autosomal
dominant gene results in a defect that is easy to identify at an
early age. When defects are seen early in life, affected dogs
can be removed from
the breeding program before passing on the trait. This
eliminates the possibility of the defect affecting future
generations.
This trait
requires 2 copies of the defective gene (1 from each parent) for
offspring to be "affected" (to exhibit the defect). Affected dogs will
pass a copy of the defective gene to ALL offspring, but not all of
the offspring will exhibit the defect. A "carrier" dog has 1
copy of the defective gene, will be clinically normal (no sign of
disorder/defect) and this dog will pass the defective gene to
approximately half of its offspring. Depending on the
genes inherited, a dog can be normal (no defective genes), a carrier
(one defective gene) or affected (two defective genes).
The following
table will illustrate the approximate percentage of normal, affected
and carrier puppies that could be expected in a given litter, based
on the genetic presence/absence of a specific autosomal recessive
gene in the parents:
|
Parents:
|
Sire |
Dam |
Sire |
Dam |
Sire |
Dam |
Sire |
Dam |
Sire |
Dam |
Sire |
Dam |
|
Normal |
Normal |
Normal |
Affected |
Carrier |
Affected |
Normal |
Carrier |
Carrier |
Carrier |
Affected |
Affected |
|
Offspring: |
100% Normal |
100%
Carrier |
50%
Carrier |
50%
Affected |
50% Normal |
50% Carrier |
25% Normal |
50%Carrier |
25%Affected |
100%
Affected |
Autosomal
recessive is the most common inheritance pattern for genetic
disorders in dogs. These genes can be hard to
identify and eliminate from a breeding program as they can remain
hidden within specific bloodlines for numerous generations.
The length of time these problems can remain hidden may lead to
breeders having a false sense of security. A breeder may
believe his dogs don't have any genetic issues because he hasn't
seen any defects. Unfortunately, the fact that no defects have
been seen does not guarantee that said dogs are not carriers for any
number of disorders. All it takes is 2 carrier dogs being
mated for any recessive disorder to rear its ugly head. New
breeders especially may be caught unprepared, but even experienced
breeders can be taken by surprise when two seemingly healthy,
unaffected dogs suddenly throw a litter in which 25% of the puppies
are defective.
If there are
only a few dogs within a particular breed that carry an autosomal
recessive gene for a specific disorder, then the incidence of
affected dogs within that breed will generally remain low as will
the risk that any particular dog is a carrier. This situation
can change drastically, however, if the gene pool is suddenly
restricted or artificially concentrated.
For example,
let's say the parent club for registered "Snockers" (my own
invention, thank you!) decided that they would no longer allow "Snockers"
OVER 15" tall to be breed to "Snockers" UNDER 15" tall.
These two groups would now be considered separate breeds and could
not be interbred. The gene pool for each of these new "breeds"
would suddenly be cut in half. Now, let's further imagine that
the under 15" Snockers happen to have a bloodline within the gene
pool that are carriers for PRA (an eye defect causing blindness).
Since the gene pool for these dogs has now been restricted, the
chances of 2 carrier dogs being mated and/or of 2 undiagnosed
"affected" dogs being mated, has drastically increased.
Suddenly the under 15" Snockers are faced with a much higher risk of
PRA carrier and affected dogs and the risk will only continue to get
worse as the concentrated gene pool produces more and more carrier
and affected dogs.
The scenario
above is only one example of how a recessive defect can suddenly
become a major breed issue. Popular bloodlines that come from
breeders who consistently in-breed or line-breed, can artificially
increase the frequency of defective genes within a gene pool.
This can also happen when a particularly popular stud dog (who
happens to be a carrier for a genetic defect) is over-bred from a
young age. Any time there is a concentration within a breed of
a particular bloodline or of a specific dog/dogs, the potential
exists that bad genes will have been concentrated along with the
good.
While it's
imperative that breeders strive to breed only the dogs with the most
desirable traits, it's also important that the decision to use a dog
for breeding be based on more than what said dog has won or
achieved. Because recessive genes can be carried by animals
that outwardly appear normal, it's vital that breeders learn as much
as possible about a potential breeding dog's ancestors. Hidden
defects can be maddeningly difficult to eradicate once they are
established within the gene pool, so follow the old rule: "an
ounce of prevention is worth a pound of cure"!
These
traits appear due to genes located on the X chromosome. Males have
a Y chromosome from their father, but this chromosome doesn't carry
much information beyond that necessary to ensure the resulting
offspring will be a male. Males will receive an X
chromosome from their mother. Females have two (2)
X chromosomes, one inherited from each parent. Sex-linked
genetic traits in males are always inherited from the dam. If a
breeding bitch is a carrier for a defective recessive gene, she will pass this
recessive gene to 50% of her daughters, who will also be unaffected carriers
(unless the sire is an affected male). 50% of the male
offspring produced by a carrier female will be
"affected". Females produced by affected males will ALL
be carriers.
The
three inheritance patterns discussed above are relatively simple and
easy to understand and identify. Defects due to genes that
follow one of the above patterns can many times be identified with
test breedings, pedigree research and careful health screening of
breeding stock. Unfortunately, other inheritance patterns can be
much more complex and the traits governed by these patterns can be
much more difficult to eliminate from the gene pool.
These other
inheritance patterns are much harder to isolate and identify as the
resultant traits are due to the interaction of
many genes. For example, the expression of some genes may be
influenced by modifying genes; or alleles at one locus may "mask" the
action of other pairs of alleles (Epistaxis); or a dominant trait may
not always be expressed due to incomplete penetrance. Traits
inherited due to these complex patterns of inheritance are known as
Polygenic traits.
While Polygenic
traits are controlled by any number of unknown genes, the gene
expression can also be influenced by factors such as breed, gender
and rate of growth. Additionally, environmental factors such
as nutrition and the amount and type of exercise a dog receives can
play a significant role in how these genes are expressed. The
heritability of polygenic traits can vary by breed as well as by
individuals within a particular breed. Polygenic
traits include temperament, personality, working abilities, height,
weight and, unfortunately, some genetic defects.
Due to the
difficulty in identifying all of the genes and the gene interaction
responsible for polygenic traits, it's very difficult to eliminate
disorders that are inherited in this fashion. To
minimize the risk of such defects within a breed, it's imperative
that breeders use all of the tools at their disposal to identify,
record and publicly acknowledge the occurrence of such defects.
The Orthopedic Foundation for Animals (OFA) is one such tool.
This public database stores information on a number of canine
issues, such as hip and elbow dysplasia, which can help breeders to
identify genetically normal and/or defective dogs. This
information can help breeders make informed breeding choices when
looking for mates for their dogs.
