DNA testing is an important new tool that breeders can use to screen their breeding dogs.  I covered the basics of DNA testing here.

Reading the title of this post some of you may be wondering, how could DNA testing ever be a curse?

Let me explain.

In the case of the single-gene simple recessive diseases, (like PRA or EIC), DNA testing has definitely been a blessing. It wasn’t too long ago that breeders had no way of predicting whether a particular mating would produce affected pups because remember these are recessive genes so just having one “bad” gene would not cause the sire or dam to be affected by (or express) the disease. However, two perfectly healthy dogs could be paired and produce affected pups.  DNA testing has given breeders a tool to guard against reproducing these diseases.

Unfortunately, with all things where you have over zealous humans involved, some take test results to the extreme in my opinion. I have heard more than once: “I breed only clear dogs and bitches together”.  I have even known breeders who have passed up keeping a superior puppy to use as part of their breeding program just because it was a carrier.  They elected instead to keep a puppy that wasn’t as nice in conformation or working ability because it tested clear.

I do not know if the breeders who breed only clear dogs do not understand genetics; think that it makes their puppies easier to sell; or think they are bettering the breed, but whatever the reason, I think it is extremely short-sighted on their part.

In the case of a carrier or even an affected dog with a single-gene recessive disease, all the breeder has to be mindful of is to breed to a clear dog. If the dog was affected, (there are still some out there because of the newness of some of these tests), breeding to a clear will produce all carriers. Obviously carriers should be only bred to clears which will produce both clear and carrier pups.  Eventually over time and some generations, it is not difficult to imagine that there will be no more affected or carrier dogs in the gene pool. The problem is that some breeders seem to want this to happen overnight and not as a result of well thought out selection.

It is even worse in the case of some of the polygenic diseases like DM (degenerative myelopathy). The DNA test for DM only provides a small clue as to whether a dog will develop the disease. The researchers have identified a gene that is present in dogs who develop the disease.  DM is a polygenic disease which means that it takes many genes and maybe some other things such as environmental factors for a dog to develop this disease.  The presence or absence of one gene is a clue, but far from definitive.

There have been dogs that have two copies of the “bad” gene but lived their whole long life without ever developing the disease.  There have also been a few (and by few I believe the number is less than 10), dogs of various breeds who have been identified as carriers and have developed the disease.  The geneticists have been pretty clear as to the limitations of the current DNA test noting that if you have a dog affected by the disease that has other dogs in its line affected by the disease, then you want to be much more careful with your breeding decisions.

But in the case of “at risk” dogs who never develop the disease, could it be something in their DNA that prevented this and if that is the case, wouldn’t you want to bring their DNA forward to future generations?

So what are some of the pitfalls of only breeding clear dogs as I see it? In breeds where there are a lot of breeding animals, maybe the impact is not great. Sure you loose some of your genetic diversity, but perhaps there are enough dogs in the gene pool to make up for that loss. However in a breed like the Chesapeake, there are limited breeding animals and limited litters produced each year. (These are the US registrations for Chesapeakes: 2012: 1999 dogs, 593 litters;  2013: 2145 dogs, 587 litters.  Not all registered dogs will be bred.)

Breeding only clears substantially limits your choices. Now you need a dog and a bitch that match up well together and are correct in conformation, temperament, working ability and clear on all health screenings.  It is not too difficult to imagine that something has to give.

But more importantly, the clear dog you are breeding to today is clear for the tests breeders are testing for today. But in the future one can imagine that the researchers will continue to develop additional and more accurate DNA tests. Maybe someday they will develop an accurate DNA test for say cancer, or kidney disease, or epilepsy.

But what if the genes that would prevent these diseases have been left 8 or 10 generations back because there was a preference to breed only dogs who tested clear for the few DNA tests that were available at the time?  Or conversely, what if by only breeding dogs which test clear for the DNA tests that we have right now doubles up the “bad genes” for say cancer, or kidney disease, or epilepsy?

A stretch maybe.  But is it?  Just something to think about.

Next in the series I will begin to tackle some some of the diseases for which DNA tests are available today.