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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