George A. Padgett, D.V.M.
Professor of Pathology
College of Veterinary Medicine
Michigan State University
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Although Irish Setters do not get Sebaceous Adenitis, this real case study in Poodles illustrates so well the need for open registries and retrospective test mating that it was used here as an example for these techniques. Irish Setter breeders (like all breeders) should look at this article and decide whether these techniques could help control genetic disease in your breed, especially with diseases like PRA, epilepsy and cataracts.
First we probably should define an open registry. An “open” registry is a registry in which all animals are identified in respect to phenotype and, when possible, in respect to genotype. All information on all dogs is available to the general public. This is as opposed to “closed” registries which are the prototype registries found in the U.S. Examples of “closed” registries are the Orthopedic Foundation for Animals (OFA) and the Canine Eye Registration Foundation (CERF). In “closed” registries the only information available to the public is on those animals which are phenotypically normal. The word phenotype means the observable properties of an animal (structural and functional) produced by the interaction of that animal’s genotype and the environment. For example, with hip dysplasia you might have abnormal (excellent, good, fair) or dysplastic (borderline, mild, severe) phenotype based upon a radiograph. It is what you observe or measure in an animal in regard to the trait in question. With sebaceous adenitis (SA), you might have a normal, sub-clinical SA or clinical SA phenotype based upon a skin biopsy.
Okay, we now have a nice distinction between and “open” and “closed” registry, but from a breeder’s standpoint, what difference does it make?
With single gene traits, especially those with late onset, such sebaceous adenitis, PRA and epilepsy, it makes all the difference in the world. An open registry allows you to “retrospectively” test mate your dogs. That is, it allows you to genotype your dogs based upon matings that may have taken place many months or even years ago. The single requirement for this is the knowledge of the genotype of one of the animals involved in the mating. This of course you can obtain from an open, but not a closed registry.
Let us give you an example of how retrospective test mating works. Since all dogs on the average carry defective genes, perhaps as many as 4-5, every mating is in fact a test mating. The problem we currently have since we do not identify “affected” dogs is that we do not know what the test mating is for. Remember, it is a test mating whether that was your intention or not. It is also a test mating even if it occurred before the dog developed the disease or before you knew the dog produced the disease.
As an example of how retrospective test mating takes place, let’s take “Charlie”, a standard Poodle. His breeding history is recorded in the pedigree in figure1.
He was born in 1982. He was radiographed for hip dysplasia, had a CERF examination and produced his first litter in 1984. He was subsequently bred 25 more times producing a total of 174 puppies. An average of 6.69 puppies per litter which is about right for a Poodle. In 1987, a bitch puppy of Charlie’s developed a skin disorder and was diagnosed as having sebaceous adenitis (SA). The first report of SA in Poodles was by Rosser et al in 1987 so the disease was not known as an entity prior to this time. A second puppy, a male in the same litter, developed clinical SA in 1988 and the breeder withdrew Charlie from public stud. Charlie never developed clinical SA. In 1989, 2 more of Charlie’s puppies, which were born in 1985, showed up with SA. Even though Charlie was clinically normal, he was biopsied for SA in 1990 and it turned out he was sub-clinically affected.
So, where are we now? Well since Charlie is affected, all 174 of his puppies are defined carriers. Does this make Charlie a “bad” dog. Not in our opinion. In fact, he is a champion and has produced 29 champions, which is 16.6% of all of his offspring. Not bad! Is Charlie’s owner a bad breeder? Not in our opinion. The disease was not known when he was first used at stud and 25 of the 26 matings he made were complete before his first affected puppy was detected. He made one more mating and was withdrawn from stud when a second affected puppy was found. There was no way the breeder could have known Charlie was affected and in fact did not find out he was affected until 1990, well after he was withdrawn from study. Charlie still has not shown clinical signs of SA; he is one of the sub-clinical cases.
So, where are we as a result of Charlie? Well, if we use the knowledge that Charlie is affected, we can do something about the disease. Twenty-three bitches have been test mated for SA and at this point in time, 6 of the bitches have proven to be carriers. How about the rest? It is probably true that the puppies from many of these matings are spread out or lost. But, if you can locate 4 puppies randomlyfrom any of these matings and all 4 are normal, the bitch is 93.7% sure to be free of the SA gene. All you have to do to establish clear dogs is to check the already born offspring. Many of the puppies are at the right age and everything. In addition, all 174 of Charlie’s puppies are defined carriers, correct? Many of them have already been bred. Thus, any mate bred to one of Charlie’s puppies has been test mated to (at a minimum) a carrier. Therefore, is you randomly evaluate 7 of their puppies and all are normal, the mate is 86.6% sure to be freed of the SA gene.
Obviously, it is not desirable to have dogs which transmit diseases like SA or PRA or epilepsy, but if you have an open registry you can use the information correctly to select dogs with a greatly reduced risk of having the gene(s) in question. At present, the only alternative to an open registry is to stumble along blindly like we have been, praying that the puppies in your next litter are normal.