Genetic testing for dogs: All about “carriers”

So you have picked out a puppy, and horror of horrors, your breeder responsibly tells you that this puppy could be a carrier for a severe disease. What should you do?

This situation will likely become more common as we identify the genes for more diseases and develop more DNA tests that can be used on our dogs.

This article is all about why, in most circumstances and where the mechanism of inheritance is clear-cut, it is perfectly safe to purchase a puppy that is or might be, a carrier for a known disease.

Straightforward inheritance

There are many diseases that we now test for in pedigree dogs. Many of these diseases are autosomal recessive in nature.  Each puppy inherits two genes for the disease; each can be either normal or faulty. Only if both genes are defective will the puppy develop the disease. The pup gets one gene from his parents to make up the pair of genes.

If the puppy inherits two normal genes, he will be in the  ‘clear’ (or unaffected).  He will never get the disease, nor can he pass it on to his puppies. But let’s look at what happens if the puppy inherits a faulty gene.

The on/off switch

The average gene in each pair usually has the power to  ‘switch off’ or override its faulty partner. This means that the dog only needs one of the pairs of genes to be expected to stay healthy.  So if the puppy gets one defective gene from his Mum and one normal gene from his Dad (or vice versa), he will still be beneficial.

He is called a ‘carrier’ because he has the potential to carry the disease on to the next generation. He will pass a faulty gene on to about half of his puppies if mated. If each of these puppies inherits a normal gene from its other parent, they too will be healthy. This is how diseases can sometimes skip generation after generation.

Affected puppies

With autosomal recessive conditions,  if the puppy gets two faulty genes, we know that he will definitely develop the disease at some point because there is no normal gene to switch it off. We call these puppies ‘affected’.   Obviously, we all want to avoid affected puppies being born, and most of us would like to avoid purchasing an affected puppy.

The benefits of testing

A responsible breeder tests his dogs and bitches for autosomal recessive diseases, evident in the breed,  and only ever mates a ‘carrier’ with a clear dog.’  Remember that the clear dog has no faulty gene and gives a healthy norma gene to every pup, so no puppies can ever get sick from this disease, even when the other parent is a carrier.

It is essential to know that if you buy a puppy from untested parents and both happen to be carriers,  a quarter of all their puppies will be ‘affected’ puppies.  Worse still, half the puppies will be affected if one parent is a carrier and one is affected. This is why buying from health-screened stock is so important.

Why mate carriers at all?

“But why is the breeder using carriers?” you say.  Why not have a policy of only mating ‘clear’ dogs to other ‘clear’ dogs.

To understand the benefits of breeding from carriers,  we need to look at the whole dog and the range of different health issues affecting each breed. Maintaining an excellent-sized ‘gene pool’ in each species is crucial to health. The smaller the gene pool,  the greater the chance of more diseases appearing.  We mustn’t eliminate perfectly healthy dogs from this gene pool for no good reason.

With the development of so many available tests,  breeding from carriers is not only possible,  but it is a  good thing because it helps keep that large and healthy gene pool. One of the beauties of widespread testing is that it allows us to breed from carriers because we can ensure they are always mated to a clear dog. The more people that test,  the more clear dogs will be available, and the bigger our gene pool will be.

With so many diseases to test for nowadays, restricting our breeding stock to dogs that are ‘clear’ for various conditions would reduce the gene pool dramatically and harmfully.

It is ok to buy a carrier.

I hope to show that there is nothing wrong with a breeder offering for sale puppies that may be carriers for a disease inherited in this straightforward way. You can only avoid a carrier puppy if you intend to find your own breeding line.   You should not discount a page if all else is right.

As tests are developed for more diseases, finding a puppy that is ‘clear’ for all of them would be a problematic and unnecessary challenge.

In summary, carriers do not get sick, at least not from the disease they carry. Carrier puppies are not substandard,  and the breeder is ideally within their rights to sell a puppy knowing that it is, or might be a carrier. She is also responsible for providing her puppy buyers with this information and, in most cases, will ‘endorse’  the registration of her puppies so that they cannot be bred unless they have been tested for the disease in question.

If you buy a carrier puppy, her faulty gene will have been switched off by the normal one and just sit there doing nothing throughout the dog’s lifetime. All you need to remember is that your dog could pass the faulty gene on, so if you ever mate your ‘carrier,’  you must mate her to a ‘clear dog’  to ensure none of her puppies get sick.