Trying to interpret your puppy or their parent’s DNA health test profile can be complicated. In this post, we have outlined some key concepts that can be helpful when trying to decide how important certain DNA results are. Many RightPaw breeders are world-class experts in the health concerns of their breed and are at the cutting edge of testing and monitoring for genetic diseases. If you are struggling to understand the DNA results presented to you or have a specific concern, reach out to your breeder and ask if they can explain the significance of the results.
What does it mean if my puppy’s parent is a ‘carrier’ of a genetic disease?
You may read in their health test results that your puppy’s parent is a ‘carrier’ of a genetic disease, whilst the other parent is ‘clear’. What does this mean and should you be worried about it?
A ‘carrier’ is an animal that has one copy of a mutated gene, and one normal copy of the gene. Many genetic diseases are ‘recessive’, which means a puppy needs to inherit two copies of a mutated gene to develop the disease. If your puppy’s parent is a carrier of a particular disease this does not mean they have the disease. It actually means they are safe from the disease because their normal copy of the gene protects them.
As long as only one of your puppy’s parents is a carrier, then your puppy is totally safe. Your puppy will only inherit a maximum of one copy of the mutated gene, and the other copy will always be normal, so they will not be affected. A carrier parent cannot pass on the disease to their offspring if they are mated with a non-carrier or ‘clear’ parent.
A carrier parent should not be mated with another carrier of the same disease as this means there is a chance their puppies could inherit both the mutated copies of the gene, one from each parent, and therefore could be affected by the disease.
So why breed from carrier parents at all? It is important to keep breeding from carrier parents in some breeds, especially where a disease has been quite common in the past and there might be many carriers out there. If we were to stop breeding from carriers altogether, it would severely limit the genetic diversity in the gene pool and we could see other in-breeding problems developing in the breed. Responsible breeders will carefully select parents so a carrier dog is always bred with a non-carrier and the puppies will be clear from the disease.
Incomplete Penetrance
Just when you thought you had your head around it, there are a couple of exceptions to the rule of never breeding two carriers together. If you are confused because your pet’s results include a result that seems to break the rule, the disease in question could be one that has incomplete penetrance.
The ‘penetrance’ of a disease is the percentage of individuals carrying the ‘affected’ gene combination, who actually display symptoms of the disease. For many diseases, this percentage will not be a full 100%, meaning there will be a number of animals who have two copies of the affected genes but are lucky enough to never go on to develop the clinical disease.
If a disease has an unusually low penetrance, so very few animals with the ‘affected’ gene combination actually tend to be affected by the disease, a breeder may choose to breed from an ‘affected’ but healthy animal or breed two carriers together.
A real-world example of this is Von Willebrand Disease type 1 (vWD-1) in the German Spitz. Von Willebrand Disease is a disease that typically causes difficulty with blood clotting, and affected dogs can experience severe bleeding after surgery or an injury. It can be life threatening for badly affected animals and many breeders screen for it in their breeding programs. In many breeds, vWD-1 shows incomplete penetrance, meaning some dogs, despite carrying two mutated copies of the vWD gene, actually don’t seem to show any clotting problems in real life.
In the German Spitz there are currently no reported cases of clinically affected dogs with clotting problems, even though we know the genes for the disease exist in their DNA. German Spitz breeders therefore, are likely to continue to breed from affected or carrier dogs, but will continue to monitor their puppies closely in case they ever do find a puppy who does seem to be impacted by the disease.