GDC (Institute for Genetic Disease Control)
PO Box 177, Warner, NH 03278 Tel. 603-456-2350
gdc@conknet.com
[GDC Copyright 2003. For permission to reprint this article contact
georgepackard@conknet.com]From GDC Newsletter, 2000
Dr. George Padgett
On DNA tests and open registries,
smart breeding and more
George A. Padgett, DVM, professor of pathology at Michigan State University, is internationally recognized as a researcher, writer and lecturer in the field of canine genetic disease. He was instrumental in the creation of GDC. His most recent book is Control of Canine Genetic Diseases (New York: Howell Book House 1998)
GDC: Almost two decades ago you were one of the early advocates in the U.S. for controlling canine genetic disease through open registries. Has you position changed?
Dr. George Padgett: I had done a paper on Malemute enchondrodystrophy (dwarfism) and then helped the western Alaskan Malemute breeders set up a registry to try to control that disease. They were one of the first local open registries in the country, and what I was saying then is the same thing I'm saying now. The only way we are going to do anything with genetic disease is to make information generally available to breeders. You have to know which dog has the genes for what disease. It's as simple as that. Even as we develop more and more DNA tests, we still have to make that information available to the people who are choosing which dogs to breed and buy.
GDC: Won't the increasing number of gene tests preclude the need of open registries?
Padgett: No, because people will need that information to make good decisions about breeding. We still have to deal with the genetic material in both dogs. First of all, you need to find out about both dogs you are breeding; testing just one dog doesn't help the breeder. The second point is that while tests for specific genes are usually 100 percent conclusive, tests for genetic markers are not. So you have to take into account the type of test. And finally, even though we have tests for single, specific genes [autosomal recessive traits] we do not have a single test for
diseases like hip dysplasia that are influenced by many genes [polygenic traits]. We are slowly making headway in developing tests for single genes, but many of the most serious genetic diseases are polygenic. It is very unlikely that we will soon have DNA tests for polygenic diseases.
You can prevent autosomal recessive [single-gene] genetic diseases every time you breed if you know what genes your dogs have. Dealing with polygenic or multifactorial traits is not so straightforward, but again, if you have the information you need, within a few generations most of the genes that are severe can be diluted to the point where they do not make much of an impact.
GDC: But realistically, if I can breed a litter of puppies free of a particular disease gene for the cost of two DNA tests, one for each parent, isn't that worth it?
Padgett: It is, of course, for that trait. But every breed has dozens of diseases, and all dogs are carrying several disease genes.
We have to realize that focusing our attention on tests for single genes may give us the feeling that we are on the edge of solving the problem, but the reality is that we will always be dealing with two sets of as many as five or six disease genes in any two dogs we want to breed.
You can breed two phenotypically normal dogs who test genetically free of PRA and get a litter of puppies with no PRA, but with a range of other genetic diseases determined precisely by the genes the parents were carrying.
GDC: Won't responsible breeders try to get as much information as they can in addition to a genetic test, and gradually develop a strong understanding of the approaches they need to take with their breed?
Padgett: Data from the AKC show that the average length of time a breeder operates is six years (two or so dog generations). That means that at any one time, the majority of people breeding dogs is at the bottom of the learning curve. You just get a breeder educated, and then they quit. Why do they quit? I've talked with a lot of breeders and the most common reason they give is that they get frustrated by their inability to keep genetic diseases out of their line. And when they quit, essentially everything they knew about genetic disease in the dogs they were involved with disappears unless they have registered those dogs in an open registry.
GDC: So what is the connection between gene tests and open registries?
Padgett: The first thing an open registry can do for you is show you whether or not you even need to test.
But beyond that, how will hundreds of breeders get access to all the genetic test information on thousands of dogs that's going to start coming? There will simply have to be centralized sources of information on both affected and unaffected dogs generally available to breeders.
To put it another way, we will need to have an open registry associated with every genetic test.
The issue is further obscured by the general perception that phenotypically normal dogs in closed registries like OFA and CERF are certified as 'breedable.'
In effect, we are trying to use information from that type of registry to breed dogs for disease control based only on phenotype. But at the same time we know that dogs can carry both single-gene and polygenic traits and still be phenotypically normal.
We can only hope that people will begin using gene tests to identify carriers among phenotypically normal dogs. As that happens, and we get direct confirmation of how many dogs are carriers, more breeders will begin to realize that the closed registry system has really almost no value in reducing genetic disease, even though that system functions very well in determining the clinical status of the dog.
So, rather than preclude the need for open registries, the more gene tests we have, the more we will see the need for open registries.
