Introduction to Genetics: a simpler explanation
All the world?s creatures have a code locked in their cells, a code that spells out their inherited characteristics. Everything from eye color to body shape and behavioral adaptations are quantified and qualified in the code. Appearance, diet, temperament, and health are embedded in the
DNA, a marvelous spiral-bound double helix of deoxyribonucleic acid and protein that carries the key to life. Dinosaurs and mastodons had DNA, plants have DNA, and amoebas and mosquitoes and snakes and monkeys and humans and dogs have DNA.
DNA is arranged into chromosomes, twisted ribbons of genetic material that carry the destiny of each individual, breed, race, and species. A gene is the DNA-plus-protein at a particular spot on the chromosome. Chromosomes occur in pairs, each of which has a gene for a particular trait at the same location. The genetic code is inherited when one chromosome from each pair is contributed to the offspring by each parent.
The number of chromosomes differs among species.
Dogs have 78 chromosomes or 39 pairs;
Humans have 46 chromosomes or 23 pairs;
Horses have 64 chromosomes or 32 pairs, etc.
Chromosomes come in different lengths and can have 100,000 genes or more in complex organisms.
A genome is a map of the chromosomes that identifies the genes for various traits. Scientists are working to build a canine genome so that structural abnormalities and diseases can be prevented through knowledgeable breeding programs. Money for the research is coming from the American Kennel Club and various breed clubs and breeders.
Genetic diseases in purebred dogs
In recent years, purebred dogs have been described as ?genetic nightmares? in the press and by those who champion the cause of mixed breed dogs in shelters. It is true that purebred dogs have many genetic anomalies ranging from those that are innocuous (long coats or unacceptable colors in some breeds, for example) or easy to repair (entropion, the turning in of the eyelid) to those that are crippling or life-threatening, such as hip dysplasia or progressive retinal atrophy.
It is not true that mixed breed dogs are free of genetic diseases due to ?hybrid vigor,? a benefit of first-generation crosses between breeds that is lost in subsequent generations. Dr. George Padgett, a leading canine geneticist, wrote in Dog World in January 1997 that mixed breed dogs can have the same genetic diseases as the breeds crossed to produce them. Padgett said that his files include information on 102 genetic defects identified in mongrel dogs, more than double the number identified in the Cocker Spaniel, one of the country?s most popular breeds.
The eradication of genetic diseases in purebreds and mixed breeds is possible, according to Padgett. However, it depends on an alliance of breeders and breed clubs to support the research, open registries to make the research available to breeders, and educational efforts to reach puppy buyers and novice breeders.
Breeders and breed clubs are the first line of defense in protecting the integrity and health of purebred dogs. Breeds recognized by the American Kennel Club, the United Kennel Club, and the American Rare Breed Association have national and regional clubs devoted to breed activities, and many of these clubs are involved in genetic research projects for breed-specific anomalies.
Discovery of genetic links and of the mode of inheritance (simple or complex, dominant or recessive) is the work of researchers, but they must have dogs to study. Breeders have made their dogs available for research in hip and elbow dysplasia; sebaceous adenitis (a skin disease); copper toxicosis; progressive retinal atrophy (an eye disease); epilepsy; cancer; and more ? not only by providing study subjects but by submitting radiographs and other diagnostic tests to various health registries.
Padgett?s Dog World article, one of a series about genetic disease, listed several of the clubs that are deeply involved in educating members about controlling and preventing genetic disease. Breeds represented by these clubs include Alaskan Malamutes, Newfoundlands, Golden Retrievers, Great Pyrenees, Collies, Boykin Spaniels, Nova Scotia Duck Tolling Retrievers, Bedlington Terriers, Cairn Terriers, Cavalier King Charles Spaniels, Doberman Pinschers, Labrador Retrievers, Poodles, Portuguese Water Dogs, Bulldogs, Bernese Mountain Dogs, and English Springer Spaniels.
This is not a complete list; many clubs have joined the battle since the article was published. Clubs have also responded to the call for funds to bankroll research into specific diseases.
Health Research Registries
Health registries evaluate diagnostic tests as normal or diseased and make records available to breeders. Some registries are closed; that is, they provide only information on dogs that pass the tests. Others are open; they provide information on all dogs submitted for evaluation and on sires, dams, and siblings submitted for evaluation.
A family picture helps breeders select sires and dams that are not only free of disease but have produced puppies in the past that are free of disease. Unfortunately, some breeders are afraid to use open registries because they don?t want other breeders to find out about problem dogs or litters.
The major health registries are the OFA, the Orthopedic Foundation for Animals; PennHip; CERF, the Canine Eye Registration Foundation, and the Institute for Genetic Disease Control in Animals.
OFA was established in 1966 to evaluate hip radiographs in an attempt to reduce the incidence of hip dysplasia. OFA is a closed registry; it lists only those dogs that have fair, good, or excellent hips. Because it charges the same fee to read all radiographs, breeders and owners often do not send in films from dogs they think will fail to make the grade.
Today, OFA also registers dogs with elbow dysplasia, slipping patellas (knee caps), autoimmune thyroiditis, and congenital heart disease in all breeds and copper toxicosis in Bedlington Terriers. OFA also works with VetGen to screen blood tests for von Willebrand?s Disease, a bleeding disorder affecting several breeds and donates money to support canine genetics programs at Michigan State University, University of Michigan, and University of Missouri.
PennHip provides and alternative method for evaluating hip health. OFA bases its decisions about dysplasia on a single radiograph, but PennHip x-rays the dog in three different positions to judge not only the presence of joint abnormalities but the amount of joint laxity (looseness), an indicator of future problems and of the propensity for passing bad hips along to offspring. All dogs are measured against other dogs in their breed and reports are available on all dogs.
Any veterinarian can x-ray a dog for OFA evaluation, but PennHip trains veterinarians to do it their way. The resulting films are not only read by PennHip radiologists, they are also submitted to OFA for additional appraisal.
CERF is a closed registry dealing only in eye problems. While many genetic tests must be done only once in a dog?s lifetime, CERF tests must be done annually. CERF officials are considering the possibility of opening the registry to record the status of all dogs submitted for testing, not only the ones that test clear.
The Institute for Genetic Disease Control in Animals (GDC) is an open registry. All dogs submitted for the diseases it monitors are placed in the data base and records of littermates and parents are provided to those who request information about a particular dog.
"Breaking the Genetic Code," partial article from Dog Owner's Guide by Norma Bennett Woolf. Canis Major Pub., 2003.
Understanding Diversity within a breed:
"Breeders underestimate the amount of diversity that can be present in a breed; even one with a limited group of founders. A molecular genetic study of the Chinook dog breed, which was reduced to four dogs in the 1970s,
showed that there was significant gene diversity and heterozygosity in the breed.
The studbook for the Thoroughbred horse has been closed for more than 300 years. However, researchers have found that on average 63 percent of the variable gene pairs are heterozygous and that 4.7 genes are potentially available to each pair. This diversity is present in spite of the fact that 95 percent of the breed traces back to a single founder male......The perceived problem
of a limited gene pool has caused some breeders to discourage linebreeding and promote outbreeding in an attempt to protect genetic diversity.
However, it is a fallacy that each dog must carry the diversity of the breed. Studies in genetic conservation and rare breeds have shown that this practice actually
contributes to the loss of genetic diversity.....If some breeders outbreed (within the same breed), and some linebreed to certain dogs that they favor while
others linebreed to other dogs that they favor, then breedwide genetic diversity is maintained."
Link to entire article:
Phenotype or clinical "Tests" or "Testing": In this context, the word test is a synonym for, or identical to, the word examination. The test or exam may be physical, ophthalmological, radiographic, by ultrasound, or a laboratory test, etc. The purpose of a phenotypic test is to determine whether a disease(s) is present or absent. In regards to a genetic disease, if the dog is affected, it tells you the genotype, but if a dog is phenotypically (clinically) normal, it does not tell you if the dog is a carrier or not. GDC, CERF and OFA "Tests" fall into this category.
PRIORITIZING GENETIC DEFECTS
George A. Padgett, DVM
Department of Pathology and The Animal Health Diagnostic Laboratory
College of Veterinary Medicine, Michigan State University
Since all dogs (or nearly all) carry some genes for genetic defects, if you wish to control the defect in an effective manner, it is necessary to prioritize them. It is necessary to prioritize them because most dogs don't carry just one or two abnormal genes; they have 4 or 5 or more. For example, we know Cairn Terriers have about 5.6 and Newfoundlands 4.8 defective genes per dog on the average. We don't have as good information on most breeds of dogs because they have not conducted an effective survey like these two have. Many breeders, breed clubs and apparently the AKC believe that the less you talk about a problem the more likely it is to go away. This is clearly not the case because as you know we have been hiding them for years and none have gone away. It takes effective and continued selection to reduce the prevalence of a genetic defect. Take collie eye for example. A group of breeders, with the help of Dr. William Yakely, reduced the prevalence of this disorder by 38% over a 3 year period in the northwest, while in the rest of the U.S. it remains a widespread problem. This same group of breeders reduced the prevalence of grey collie syndrome to negligible levels a couple of years earlier. There are other examples like the Portuguese Water Dog Club that made major inroads on hip dysplasia and all but eliminated their storage disease problem in a few short years. The Malamute Club of America greatly reduced the prevalence of malamute dwarfism albeit there was some disagreement on what to do with carriers. So it is clear that genetic disease can be controlled, but they cannot be controlled by hiding them. In fact, hiding the defects, rationalizing them, minimizing them, and in some cases flat out lying about them is causing the puppy lemon laws like the one recently passed in Florida to come into being. No sane breeder and no one interested in purebred dogs, like me for instance, wants to see this happen, but folks whether we like it or not, it is happening.
Can we do anything about it? Of course, but not by sitting on our hands, keeping our mouths shut, closing our eyes, and letting our dogs go blind, be crippled, have fits, and other characteristics which make them unfit to be show dogs, hunters, obedience animals, and most important, good pets and companions. Breeders love their dogs (at least most of them do), and the people they sell them to bond with them and they become part of the family. This is what we want to happen with dogs, we love them, they love us and provide great pleasure and satisfaction in our lives. Is it any wonder that people are upset when their dogs go blind or become crippled or develop a disease that may cost 2, 3 or 4 times the original cost of the dog? Not to my mind. I believe they have a right to be upset and the buyers do to.
It is clearly time for breeders, breed clubs, the AKC and the veterinary profession to come to grips with the problem, both to preserve our integrity and the health and well being of our canine friends.
If 40 to 50 percent of our dogs have a defect themselves and in addition carry 4 or 5 genes for other deleterious traits, isn't the situation almost impossible to resolve? The answer to that is clearly yes, unless you do something to put order in the picture. We must prioritize the traits and work on those which harm our dogs first and put on the bottom of the list those things which do little harm or which we can readily correct. We have to quit thinking like we did in 1920 and look at the real situation as it is in the 1990's - 2000's. I hate to tell you this, but all dogs are not perfect, and since that is true, we have to decide what we can live with and what we cannot. Why is it that we can cut the tails and ears off a dog and show it, but can't repair an inguinal hernia and show it? They are both (for the most part) for cosmetic purposes. Who makes rules that requires an inguinal hernia to become equivalent to PRA or cataracts or malamute dwarfism in terms of selection? The AKC needs to reconsider some of their rules and adopt a more sensible and realistic approach to the realities of nature and of dog breeding.
However, given that the situation of dog breeding is as it is today, how do you solve the problem?
Step one - Since every time you breed dogs you get whole dogs, not just eyes or hips or elbows or hearts, you should always look at the whole dog first. The whole dog is composed of somewhere between 10,000 and 110,000 genes; we are not sure of exactly how many, but it should be obvious that it is easier to control 1 or 2 or even 5 genes than it is to control 100,000. Therefore, you must pick a dog (or dogs) for breeding that will allow you to accomplish your goal(s), be it conformation, hunting, obedience, or the production of good pets. If your goal is to win at conformation, it will do you no good to produce an animal with superb hips if it doesn't win in a 100 times out. You will not be satisfied with the dog even though it is healthy. So you must pick the whole dog first. Of course, it will be best if you can select 3, 4 or even 5 dogs that fit this category to allow secondary selection parameters to be introduced. However, with a few exceptions, this is not essential, it just makes life easier.
Step two - There are some traits that override the primary selection parameters (i.e., conformation, hunting and working abilities). In my opinion, one of these and perhaps the only one is temperament. It should be obvious to us that are involved with purebred dogs that the public (including legislators) are upset about the potential for physical harm that exists with dogs. We need to come to grips with the fact that many people, perhaps most people, cannot handle some of the dogs produced by breeders. They are not familiar with aggressive dogs and do not have the knowledge or skill to handle them. This, of course, causes many problems, hence the specific breed and all breed legislation that has swept the country. But, although the problem may be more severe with some breeds than with others, in reality it applies to all dogs. I have seen massive efforts by the AKC, specific breed clubs, and all breed clubs to modify the proposed legislation to bring it into a more realistic evaluation of and correction of the problem. These meet my hearty approval. However, I have seen little in the way of efforts by any of these organizations to get at the heart of the problem, that is, the dogs themselves. The number of dog bites tends to be proportional to the size of the breed, although perhaps not completely so. There is also a clear variation in the intensity and severity of the attacks. However, it should be obvious that such breeds as Golden and Labrador Retrievers, Collies and Poodles (etc.) bite more people than Bull Terriers. This is not because they are more aggressive, but because there are more of them, lots more. This, of course, applies to all breeds, not just the 4 mentioned above. We need to make it clear in a straight-forward and unambiguous way, that it is unacceptable to breed dogs with a poor temperament. We can argue ad nauseam about what poor temperament is, but we cannot accept it in our breeding stock, no matter how good their other characteristics are.
Although I am not a behaviorist (nor do I intend to become one), I offer this opinion realizing that it does not cover all aspects and that there are some extenuating circumstances in some cases. Dogs that bite people and other dogs, dogs that continually growl and exhibit aggressive behavior towards people and other dogs, dogs that more than once upset or interfere with conformation shows or filed, obedience and other trials (and the first time should be documented) should not be bred independent of all other factors.
The AKC and specific breed clubs, as well as all breed clubs, should instruct their judges that such behavior is unacceptable in dogs and these dogs must be excused from the ring or trial. It is most important that they be excused on the first offense, because we cannot document the second offense without the first being in place. Those dogs exhibiting poor ring or show behavior twice should be permanently excused. Those judges that cannot or will not excuse dogs for poor behavior should themselves be excused from the ring, permanently.
Over a period of time, if we continually select against poor behavior, and if all of our clubs and judges exert the peer pressure necessary to make this a standard selection parameter against poor behavior, we will again regain the trust of our dog buying public.
Step three - We have now selected our breeding stock, that is a dog (hopefully dogs) that fulfills our good citizen requirement as well as our requirement for a winning phenotype, be it in conformation, hunting, obedience, working or serving as a good pet and companion.
A good breeder with some experience with dogs in his/her line or kennel should know the diseases that are present in his/her stock. There may be some excuse for a new breeder or one that has produced only 1, 2 or 3 litters not being aware of what his/her stock carries, but there is no excuse for the breeder that sold him/her that stock not telling her/him what disorders are involved with the line. Good breeding practices require ethical behavior. Be that as it may, if you know what's in your stock you know what to select against, which means that what is good and correct for one line may be inappropriate for another. So it should be understood that all breeders will not be facing exactly the same problems at the same time.
Since there are an average of 4 or 5 defective genes per dog and since it is difficult to select against more than one (or perhaps, if you are lucky, two) trait(s) at a time, you need to prioritize the disorders. In table 1 I present my opinion of a prioritization scheme, a hierarchy of disagreeableness of genetic traits. As you look at the table, you will see that a given trait may fall into more than one category. The more categories it fits the less desirable it is so the more severely it should be selected against. This table is not intended to be a list of the only diseases you should select against; they are examples and there are hundreds of other diseases that belong in one or more of the categories. Further, there are no breeds of dogs that are exempt from this table. Just because I have not mentioned a disease that occurs in your breed does not mean that no such disease occurs in your breed. Further, just because you do not have a severe trait in your line or kennel does not mean that you should not select against the less severe traits.
Once you have selected your breeding stock and know what traits occur in your stock and have prioritized them, you now know what to look out for in the mate. It is true that it is difficult to determine what traits will be present in your selected mate because breeders tend to avoid telling you or minimize them or outright lie about them, but you now have a starting point and can ask appropriate questions, which will help you block the occurrence of a trait or eliminate the gene from your line or kennel. Most of the dilemma about identifying dogs which carry genes for specific traits could be eliminated by open registries. An open registry identifies dogs that are affected with or carry specific defects as wall as those dogs that are pheno-typically and genotypically normal for such traits. Examples of functional open registries that exist now include: The West Highland White Terrier WATCH Registry, which is endorsed by the WHWTCA and registers CMO, Legg-Perthes, hip dysplasia and Globoid Cell Leukodystrophy; The Portuguese Water Dog Club of America's Storage Disease registry, Georgia Gooch's Labrador Retriever PRA registry. The Institute for Genetic Disease Control's (GDC) hip and elbow dysplasia and OCD of the shoulder, elbow and hock registry. The GDC also registers poodles with sebaceous adenitis under sponsorship of the Genodermatosis Research Foundation and approval of the Poodle Club of America.
HIERARCHY OF DISAGREEABLENESS OF A GENETIC TRAIT
1. DISORDERS THAT CAUSE PAIN TO THE ANIMAL (examples: glaucoma, cranial mandibular osteopathy, hip dysplasia, entropion, portocaval shunts, dermatomyositis, cancer)
2. DISORDERS THAT DISFIGURE, MAIM OR OTHERWISE RENDER AN ANIMAL NONFUNCTIONAL (examples: English Pointer dwarfism, cataracts, retinal dysplasia and detachment, Malamute chondrodystrophy, PRA, deafness)
3. DISORDERS THAT KILL (examples: malignant histiocytosis, inherited kidney disease, Anasarca, globoid cell leukodystrophy, osteosarcoma, Portuguese Water Dog storage disease)
4. DISORDERS THAT REQUIRE TREATMENT FOR THE LIFE OF THE ANIMAL (examples: Grey Collie syndrome, diabetes, epilepsy, copper toxicosis)
5. DISORDERS THAT REQUIRE SURGICAL CORRECTION FOR THE ANIMAL TO SURVIVE OR LIVE A RELATIVELY PAINLESS LIFE (examples: esophageal achalasia, ventricular septal defects, elbow dysplasia, distichiasis)
6. DISORDERS THAT ARE DIFFICULT TO CONTROL (examples: multigene traits, late onset traits such as sebaceous adenitis, subaortic stenosis, osteochondritis dissecans)
7. DISORDERS THAT ARE READILY TREATABLE AND RESPOND WELL TO THERAPY (example: hypothyroidism)
8. DISORDERS THAT REQUIRE ONE TIME SURGERY THAT IS HIGHLY SUCCESSFUL AND PRINCIPALLY COSMETIC (examples: inguinal hernia, umbilical hernia, mild tooth disalignment - these are equivalent to tail docks and ear clips, unilateral cryptorchidism)
9. DISORDERS THAT PREVENT AN ANIMAL'S USE FOR THE PURPOSE FOR WHICH IT IS BRED (examples: Samoyed albinism, correct color, mild bite, dentition and gait abnormalities)
TO TEST OR NOT TO TEST
The term "Test" is a confusing one because we use it in varying contexts which gives the word various meanings. Therefore, I shall begin by defining some of the contexts and thus meaning of the word. We will later discuss several aspects of these contexts.
1. "Test" Mating: A test mating is a mating in which you are trying to maintain or improve the quality of the offspring in relationship to the parents and/or line and to specifically determine the genotype of one of the parents and, as far as possible, the offspring.
2. Retrospective "Test" Mating: A retrospective test mating is a mating which has taken place at sometime in the past and for which information has now (or currently) become available which allows you to make a specific determination about the genotype of one of the parents and, as far as possible, the offspring.
3. Random "Test" Mating: Random test matings are matings which allow you to determine the genotype of one or both parents based on knowledge of the prevalence and/or incidence of specific genetic diseases in the breed as a whole. This is particularly useful with males, especially "matadors."
4. Screening "Tests": These are tests that are used more or less routinely to determine whether or not a disease is present in an individual. In human medicine, a common example would be thyroid testing at birth to rule out cretinism, and in canine medicine, thyroid or VWD testing to rule out these diseases. These are generally laboratory tests and they are used for diseases that have a high frequency in a breed.
5. Phenotype or clinical "Tests" or "Testing": In this context, the word test is a synonym for, or identical to, the word examination. The test or exam may be physical, ophthalmological, radiographic, by ultrasound, or a laboratory test, etc. The purpose of a phenotypic test is to determine whether a disease(s) is present or absent. In regards to a genetic disease, if the dog is affected, it tells you the genotype, but if a dog is phenotypically (clinically) normal, it does not tell you if the dog is a carrier or not. GDC, CERF and OFA "Tests" fall into this category.
The problem with the word "Test" is that many times, if not most times, you hear the word you cannot determine for sure the context it is used in. For example, someone tells you their dog has been "tested" clear for CMO. Do they mean their dog has had its jaws palpated and the owner or their vet found no bumps or swellings and their puppy shows no pain? Do they mean their dog was radiographed at 10 months of age and no evidence of CMO was present? Do they mean their dog was bred to a mate that had CMO, produced 4 pups all normal, and they now know their dog is 93% sure to be normal genetically for the CMO gene? Do they mean their dog was bred to 4 (or 8) different mates (with no knowledge about their phenotype or genotype), produced 12 (or 24) puppies, nobody told them they had a puppy with CMO and, therefore, their dog has to be free of CMO? Do they mean no one has told them they have ever had a case of CMO in their family or line? There are clearly different meanings and different levels of precision which are involved in the potential interpretations of the statement "my dog has been tested clear for CMO".
Now let us look a little more closely at what is involved in the various definitions of "test" or testing that were described above and how they relate to controlling genetic disease. But, before you bother to read any further, you need to make some personal and professional (as a breeder) decisions about the following:
1. Are there problems with genetic disease in your breed?
2. Is it your goal to breed healthy, winning dogs?
3. Do you think a person purchasing one of your puppies should expect to get a healthy dog (is a healthy dog a reasonable expectation)?
4. Do you believe it is possible (perhaps with a few changes in the way things are done) to breed healthy dogs?
5. Are you willing (if a fair number of other breeders do, even though all will not) to discuss genetic disease in a reasonable manner and identify (your) dogs with genetic defects?
IF YOUR ANSWER TO ANY OF THESE QUESTIONS IS NO, STOP! THROW THIS ARTICLE AWAY IMMEDIATELY, BECAUSE THERE IS AN OUTSIDE CHANCE IT WILL CONTAMINATE YOU.
TEST MATING - a type of "test":
Test mating is a technique that is embraced by some, vilified by others, and is not understood by most. There are ground rules for a good test mating program, and if they are followed, it will not be, cannot be, the "downfall of the breed" as predicted by so many of its detractors. Test mating is a prospective technique; that is, you are trying to determine what disease will occur or will not occur as a result of a particular mating. In other words, you are looking at the future. An essential requirement of a test mating is that you must know the genotype of one member of the mating which makes the opposite member the test mate (i.e. the one whose genotype you are trying to determine, and it can be either a dog or a bitch). In addition, there is a breakpoint in test mating. The breakpoint is determined by the average number of puppies produced in a litter. The breakpoint is about 5 puppies per litter; that is, if you average 4 or less puppies per litter, you should use an affected animal on your test mate, and if you average 5 or more puppies per litter, you can use either an affected or a carrier animal on your test mate and be effective. This difference is designed to allow (particularly a bitch) reproductive time to produce the normal litter(s) which is your goal (given that an average number of litters for a quality bitch is 4). It can be readily and clearly shown that you produce an identical number of carrier puppies whether you use an affected or carrier animal to test against. Further, to genetically clear a dog to the 99% levels using a dilution system, you also produce an identical number of carriers. If you just randomly mate your dogs, the production of carriers goes on forever.
Ground Rules for Test Mating:
1. Never use an affected or carrier dog in your test mating program which does not have an equal or better phenotype than the dog you are testing (otherwise you will go backward and will wt achieve your goal of winning).
2.. Check all offspring using the appropriate diagnostic techniques at the appropriate age for the disease in question in order to identify the genotype of the dog you are testing.
3. If you used an affected dog in the test mating, all offspring will be defined carriers (i.e. they have to be carriers), and if a carrier was used, each puppy will have a 50:50 chance of being a carrier. These puppies should be clearly identified as such when they are sold and arrangements should be made to assure they will not be used for breeding under normal circumstances (the defined carriers may be useful in a test mating program).
Retrospective "test" mating:
The discussion of test mating applies equally well here; the only difference being that the actual mating took place before you knew the genotype of one of the animals involved in the test mating. This subject will be covered much more extensively in a future article.
Random "test" mating:
In order to use random test mating, you need to have a reasonably extensive knowledge of the incidence or prevalence of genetic disease within a breed. Once you have that knowledge, you can then calculate the number of carriers in the general population. Once you know the number of carriers, you can calculate the number of random matings which would be needed to "test mate" a dog or bitch to a given degree of risk of having or not having a gene for any of the diseases which occur in the breed. At the present time, to my knowledge, there is sufficient information on CMO, Legg-Perthes, and hip dysplasia in Westies to use this technique. In Cairn Terriers and Newfoundlands there is sufficient information on about 30 disorders in each breed to use this technique. This is particularly useful for matadors (a matador is a dog which has produced on the order of 50 or more puppies). I will discuss random test mating much more extensively in a future article. It should be clear that random test mating evaluates matings that have already taken place and from this standpoint it is like retrospective test mating. In order to use this effectively, one needs to keep track of as many offspring as possible.
Screening tests are generally used to evaluate a dog for diseases that have a high incidence or prevalence in the breed, or a disease in which all dogs are at risk. They may be inherited or not inherited. An example of a noninherited disease is brucellosis and both a dog and bitch should be screened for this disorder prior to breeding, particularly if the proposed mate
is an "outside" animal. Evidence that their dogs have been recently "tested" for this disease should be made available by the owners of both the stud and the dam.
For a genetic disease, the ground rules are a little more complicated. Every major stud (every matador), including those dogs which have the potential or likelihood to become a matador, should be "tested" (screened, examined) for the diseases that tests are readily available for. The reason of course is that matadors become matadors because they are used mainly in outcrosses. Outcrosses, unlike inbreeding, spread a genetic disease throughout a breed. Examples of tests that should be used are annual CERF exams for eyes, GDC or OFA radiographs for Legg-Perthes (and on the same radiograph, hip dysplasia), and CMO, as well as, VWD and thyroid testing. It is a little different with bitches because they would rarely be classed as matadors in any breed. Nevertheless, if they are quality animals, they may influence a significant number of descendants as the maternal head of a line. Any quality bitch, like a matador, should be screened for the major problems because of this maternal influence.
As a person who is looking to breed a cairn (or any other breed for that matter), you might think "Hey Doc, that's nice for you researchers to decide how I should spend my money, but I'm only going to breed this bitch once" and that the foregoing is a waste of money. If you are talking about the type of dog that should be bred only once (if at all) you are probably right. But, what I am talking about for screening tests are significant animals. Those dogs and bitches that have, or are likely to have, a significant genetic impact on a breed.
Aside from the obvious information, screening tests provide for the animal tested; screening tests also provide information that may well be useful later on for retrospective or random test matings. Remember, screening tests only give genetic information on the animal tested if they are positive. If they are negative (i.e. the animal is clinically normal), they do not tell you if an animal is a carrier or not.
Phenotype tests or clinical tests:
Phenotype or clinical tests should obviously be used on any animal displaying symptoms that need to be diagnosed as one disease or another. They are obviously needed for both genetic and nongenetic diseases. Aside from animals that are displaying symptoms of a disease, clinical tests in regard to genetic disease are most useful when the results are entered in a registry and those results are made available, whether they are positive or negative, to people who are considering the dogs for breeding. If we wish to control genetic disease in dogs, we not only need to "test" our dogs, but we need to register them in an open registry in order to make the information available for proper use. Open registries will be discussed in a future article.