Winter 2016 Saber Tails 41
Petit Basset Griffon Vendéen Club of America
What Is a Carrier?
‘Carrier’ is the term given to an individual (of any species)
that carries a single copy of a recessive mutation that is
associated with a specific inherited condition, usually an
inherited disorder. An individual will only suffer from a re-
cessive disorder if it inherits two copies of the causal mu-
tation, one from each parent. If it inherits a single copy of
the mutation it will remain healthy but will pass the muta-
tion on to about half of its offspring.
Breeding with Carriers
Once a specific disease mutation has been identified a
DNA test can be developed that enables the identifica-
tion of non-symptomatic carriers. Knowing which dogs
carry the mutation and which don’t (the so-called ‘clear’
dogs) enables breeders to make sensible choices about
the dogs they mate together. All dogs can be safely bred
with provided at least one of the mating pair is clear of the
mutation (see Table below). Breeding dogs that will never
develop the condition should obviously be the priority
for all conscientious breeders and the desire to eliminate
a disease-associated mutation from a breed should there-
fore be the long-term goal. But the instinct to choose
only clear dogs to breed from, as soon as a DNA test be-
comes available, may not always be a sensible choice and
the rest of this document discusses why.
If carriers are prevented from breeding, the opportunity
to pass the rest of their genetic material to the next gen-
eration is also lost and the genetic diversity of the remain-
ing population is thus reduced. It is worth remembering
that there is a clear and well-established link between the
genetic diversity of a population and its overall health,
and that breeding closely related individuals tends to lead
to the accumulation of deleterious recessive mutations in
the population. This is due to the fact that an individual is
more likely to inherit two identical copies of a mutation if
its parents share common ancestors than if they are unre-
lated, and the more common ancestors the parents share
the greater that chance is.
It is also worth remembering that the disease mutation for
which there is a DNA test is not the only mutation a carrier
has. Every human, on average, carries about 50 recessive
mutations and there is no reason to believe the average
dog won’t carry a similar number. So the only real differ-
ence between a clear and a carrier is the single mutation
that can be tested for. Both dogs will both carry around
49 other mutations that the breeder doesn’t know about
and can’t test for. If carriers are not bred from and clear
dogs are used extensively then there is a real risk that oth-
er mutations will increase in frequency in the breed and
new inherited disease(s) could emerge.
There is no reason why the eventual elimination of a dis-
ease mutation from a breed shouldn’t be the goal, once a
DNA test for that mutation becomes available. But, pro-
viding all breeding dogs are tested for the mutation prior
to mating, the breeders can take their time and ensure
that desirable traits are not eliminated along with the dis-
ease mutation and that the genetic diversity of the breed
is not reduced.
Mutation Frequency
The speed with which the mutation can be eliminated
depends on several factors, including the frequency of
the mutation, the population structure and the rate of
POAG DNA Testing
and Recommendations Regarding
Breeding Practices Based on Results
Written by Cathryn Mellersh, Animal Health Trust
November 2016
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