by John E. Dillberger, DVM
Reprinted from the January/February 2012 Claymore.
In response to last month’s column, I received a note from Scottie Sterrett about the example I used to illustrate genetic testing. The point that she touched on is so important that I wanted to share her letter and my response with everyone. I hope this will stimulate further conversation about the issues that genetic testing raises for all of us.
Dear Dr. Dillberger:
Thank you for another clearly written and most illustrative article on Deerhound health in the November/December Claymore. I have to say you completely lost me with the suggestion that puppies with your exemplary “green gene” could be identified and “culled” shortly after birth. If this is this the best we can expect from genetic science, I’ll go back to divination and wizardry, techniques a bit more proven.
Thanks for your feedback. It’s good to know that people read my columns. Your remark about divination and wizardry made me smile. I always knew that successful breeders practiced a little witchcraft.
Unfortunately, I am afraid that what I wrote is the best we can expect from genetic science, at least in the situation that I made up. And I created a situation that was simple compared to what I think reality will become in our lifetimes.
In my example, I used a make-believe disease called chlorophyllitis caused by a fictitious green gene. But for this letter, let’s use a real disease, like osteosarcoma, and let’s imagine the following:
- A research group has identified an osteo gene in Deerhounds. It is located on one of the regular chromosomes that come in pairs, rather than on one of the sex chromosomes.
- A genetic test exists that can tell if a dog has 0, 1, or 2 copies of the osteo gene.
- Deerhounds with two copies of the osteo gene always develop a highly malignant form of osteosarcoma by the time they are 5 years old, but Deerhounds with only one copy or no copies of the osteo gene do not. In other words, the osteo gene is recessive, and carriers (dogs with one copy of the gene) do not get the disease.
- The osteo gene is fairly widespread in the Deerhound population, with 50% of dogs having at least one copy.
In this situation, it is likely that some very good dogs with highly desirable traits will be carriers of the osteo gene. The question for all of us is what to do with such dogs when it comes to breeding. I think we have four choices: not use the dog, mate the dog to one that has no copies of the osteo gene, mate the dog to another carrier, or mate the dog to one that has two copies of the osteo gene.
Personally, I don’t believe that our gene pool is so big that we can afford to throw away a very good dog, so I would reject the first option. Being a good steward of a rare breed requires that one use the very best animals for breeding, even if they aren’t perfect.
I also don’t think we need to breed a carrier to a dog that has two copies of the osteo gene, so I would reject the fourth option. That is because, in my example, half of the world’s Deerhounds are free of the osteo gene and at least some of the other half are carriers that have only one copy. Given that, I believe one could find a suitable mate without having to use a dog that has two copies of the osteo gene. But take note: the situation might be different if the osteo gene were more prevalent. For example, if 80% of the world’s Deerhounds had the gene, we might choose to mate a carrier to a dog with two copies of the osteo gene, knowing full well that half of the pups, on average, would have two copies of the gene (like their one parent) and would be destined to die young of cancer.
I think the second and third options are perfectly sound. The second option—mating a carrier to a dog with no copies of the osteo gene—insures that one would not produce any pups with two copies of the gene; that is, one would produce no pups destined to die young of cancer. Such a mating will, on average, produce equal numbers of pups that are free of the osteo gene (like one parent) and pups that are carriers (like the other parent). With luck, one of the pups that is free of the osteo gene will have some of the best traits of the parent with the osteo gene. Such a result would be a home run. And if that isn’t the case, then at least the Deerhound population is no worse off that when it began in terms of the prevalence of the osteo gene.
The third option—mating a carrier to another carrier—has the chance of producing one or more pups that have two copies of the osteo gene. I don’t consider this unethical, provided the breeder commits to test the pups for the osteo gene shortly after birth and provided that she decides in advance what she will do with any pups that have two copies. Those pups are fated to die young of cancer. I would not choose to invest my heart, time, and money into a dog with such a death sentence, nor could I in good conscience give the dog to someone else, even if they said they were willing to accept the dog. Therefore, I would cull such pups, just as I would cull any pup with a serious physical defect. Other breeders may feel differently, and I respect that. My point is that any breeder who undertakes a mating such as I have described must think through this situation and reach a decision with which she can live. And that thinking should occur before the mating, not after the pups are born.
Although I didn’t say so in my column, it is important to remember that a carrier-to-carrier mating has an equal chance of producing an even better dog than the sire and dam and that is free of the osteo gene. That is one reason a breeder might decide to arrange such a mating.
Of course, my example is ridiculously simple. The situation we will soon face is more complicated by far. We will have half a dozen genetic tests for different diseases. Sometimes the genes will be sex-linked. Sometimes they will be recessive, but other times they will be dominant. Sometimes the genes will be rare in Deerhounds, but other times they will be common. Sometimes the genes will “cause” the disease, but other times they will only influence a dog’s chances of getting the disease, or the form of the disease that the dog gets, or the age at which the disease occurs, or how the disease will respond to treatment. It is unlikely that any Deerhound will be free of all the disease genes for which we can screen.
Please don’t despair at my vision of the future. The situation I describe really will not be a new one for breeders. Breeders have always had to choose among dogs with different mixes of desirable and undesirable traits. Genetic testing simply gives breeders the ability to choose among dogs with different mixes of a few desirable and undesirable genes, as well as desirable and undesirable traits. Indeed, it might be best to think of the genes for which we can screen as simply a few new traits to consider when it comes time to plan a mating.
With best regards,