1. PLL exists in the Norwich genome.
2. There exists a simple inexpensive DNA test for PLL.
3. Responsible breeders perform genetic tests on their breeding stock.
In conclusion, responsible Norwich breeders DNA test their breeding stock for PLL.
That really is all that one needs to know.
Currently we know of one affected dog who has 7 carrier descendents, plus a carrier-sister of the affected dog who has one identified carrier-descendent. In addition there is a missing OFA number (NT-PLL32) on the list of Norwich who have been PLL tested - which indicates that the owner did not give permission to have negative results made public - so this dog is either a PLL-carrier or PLL-affected.
This PLL mutation did not appear spontaneously. Both parents of the affected dog must have been at least PLL-carriers. And the sire of the affected dog was a popular sire - whose progeny are now on both sides of the Atlantic.
So concentrating on the descendents of the affected dog is misleading. This PLL-mutation most likely entered the Norwich genome with one or more of our founder dogs, since PLL exists in almost all the terrier breeds. So the PLL mutation could be in any Norwich. The only way to know for sure is to DNA test.
Let me say this again - if you do not DNA test, then you do not know the PLL status of your dogs.
Perhaps you mean that PLL is not a SERIOUS problem where we have large numbers of Norwich going blind from PLL. I agree that this is the case.
However the fact that the PLL mutation is in the Norwich genome means that a popular sire who is a PLL-carrier can spread the mutation widely and we could end up with large numbers of Norwich with PLL. This is not some hypothetical scenario ... this is what happened with the miniature bull terriers. (It got so bad in England that the stud book for miniature bull terriers had to be reopened to allow a bull terrier cross to fight the problem of PLL.)
So doesn't it make sense to PLL DNA test and know that you are not breeding a PLL-carrier to a PLL-carrier? Or widely breeding a PLL-carrier and spreading the mutation?
I will say it again - if you do not test, then you do not know.
Personally I do the "happy dance" when a DNA test is found to be relevant to Norwich. Prior to a DNA test the only tools that I have are interpretive tests and culling from my breeding stock, with the consequent loss of genetic diversity. With a DNA test, the problem can be considered as a minor fault, and a carrier who has superior merit can be bred and its puppies DNA tested and appropriately placed.
I know the PLL status of all the dogs in my house, and - since the PLL DNA test was found to be relevant to Norwich - the PLL status of all the dogs that I have bred. I encourage other breeders to do the same.
I respectfully suggest that *all* breeders should PLL DNA test their breeding stock. This PLL mutation did not start with the known PLL-affected dog.
Perhaps an example will make this clearer. I hope you will not mind if I use your kennel as an example.
The sire of the PLL-affected dog had to have been at least a PLL-carrier. So in the absence of further information, there is a 50 percent chance that the paternal grandfather of the PLL-affected dog was a PLL-carrier. This paternal grandfather was "Laetans Ebenezer" who is in the pedigrees of many of the dogs from your kennel.
Since these dogs have an ancestor who is a (possible) PLL-carrier, I conclude that it is possible that some of your dogs are PLL-carriers. Without testing, we do not know.
I can do the same sort of calculation with any kennel; the only thing that might change is the probability that the ancestor was a PLL-carrier.
I used the relationship feature of my online pedigree database.
I put in the known PLL-affected dog "Jerusalem Chutney" and the dog I was interested in, and found their nearest common ancestor.
We do not know the founder dog or dogs who were PLL-carriers, nor the line of descent of the PLL-mutation that led to the PLL-affected dog. All Norwich terriers - if you go back far enough - have a common ancestor.
Aside - And you do not have to go back very far. I recently was looking for the most unrelated sire for a bitch that I want to breed. Looking through all the AKC registered males in my pedigree database who could possibly be available for stud, the most unrelated sires were fourth cousins; i.e., they had a great-great-great-grandfather in common.
It is conceivable that this common ancestor was a PLL-carrier ... and so the PLL-mutation may have come down to any Norwich. Thus the only way to know for sure that you are not breeding a PLL-carrier to a PLL-carrier is to PLL DNA test (unless the PLL-status is already known by "clear by descent"). If you do not test, then you do not know.
I will say it again. *Every* Norwich could be carrier of the PLL-mutation. The only way to know for sure is to do a simple inexpensive DNA test. If you do not test, then you do not know.
AKC Breeder Of Merit
5 October 2014