Why You Should Know More About Your Breeder
© 1999 Bernd Fritzsch, Ph.D.
Dogs, like humans, have approximately 100,000 genes that guide their development from egg (and sperm) and bring them through adult life. Like books, genes are composed of letters (only four) strung together in meaningful groups known as genes. In dogs, like in humans, with which dogs have about 85% (85,000) genes in common, many of these genes are mutated. Mutation means that the sequence (and thus the function) of the genes has been altered: instead of a meaningful word like ‘breakfast’ it will have an altered spelling like ‘brefkast’. Luckily, most of these mutations are minor and those that have an effect are mostly recessive. Recessive means, if a dog gets a good gene from one parent and a mutated gene from another parent, it will be healthy. However, if such a dog with one normal and one mutated gene will be mated to a dog that also has at least one mutated gene (best chance to achieve this is to cross with a sibling, hence inbreeding will uncover genetic defects), about 25% of the offspring will have both genes (one form each parent) that are mutated. Whatever problem is caused by the mutated gene will only then become apparent. Unfortunately, 50% of the offspring will be carriers (that is having one mutated gene) and only 25% will have no mutated gene.
Logically, one would like to find ways to select those dogs that do not carry the mutated genes. Traditionally there has been only one way: breeding the dog in question with an affected dog. If the dog in question is a carrier (having a mutated gene) 50% of the offspring will show the defect caused by that defected gene. If, however, the dog in question is not a carrier (has no mutated gene) all the offspring will be healthy but carry one defected gene from the affected dog. If a mutation has a very late onset, such as Alzheimer's disease, the dogs in question will be past their reproductive age before the defect becomes apparent. Clearly, this approach is not a very feasible one.
From this it follows that a breeder has to be very rigorous in the selection of the breeding stock. Moreover, if untested animals have to be used there will be a spread of whatever genetic disease they may carry. Worst of all, if unproven studs are used excessively in small dog populations, as is typically the case in rare breeds, their genetic diseases can spread very fast throughout the entire population. It will usually take one or more generations to find out what they have carried as more or less close inbreeding (sometimes also referred to as line breeding) will eventually generate offspring in which defective genes from both parents are combined in one or more offspring. One of such cases is the spread of epilepsy in certain sighthounds. Once established in a small population of a rare breed there is not much that can be done traditionally to get out of this genetic hole.
But things are beginning to change and there is new hope to overcome this problem. Let us consider one example, von Willebrand’s disease, a disorder of blood coagulation somewhat similar to hemophilia. Affected animals produce up to 95% less of the von Willebrand factor and therefore can not coagulate blood as effectively as unaffected dogs. In some dog breeds of German descent (like the Doberman pinscher) more than 60% of the dogs are affected. Selecting the 20% or less dogs that are not carriers of this mutated gene requires identification of the gene in a technical process called genotyping. What this process does is identify any carrier and thus allow the exclusion of that dog from future breeding irrespective of the degree to which the mutation affects this very dog. Thus, any disease for which a gene has been identified can be used to screen dogs of a planned breeding for the presence of that mutated gene. In essence, using this strategy, we should be able in the next century to eliminate many of the mutated, disease causing genes that have inadvertently been accumulated in our dog populations. As technique progresses we will soon be able to screen all 100.000 genes of each dog for known disease causing mutations prior to breeding. We are already using this technique, called genetic microchips or microarrays (with up to 30.000 genes simultaneously identifiable) in our mouse breeding program at Creighton University to screen for carriers of certain mouse mutations which we like to breed to study the effects of the mutated genes.
While in many cases of genetic diseases in dogs nobody can be blamed directly (how can you avoid doing something you are not able to see?) there are certain conditions which should be avoided in order to minimize the risk of selecting carriers for a breeding program. Clearly, diversity of breeding is one way of safeguarding against a single (possibly defected) genome infiltrating an entire population. Thus far I have dealt with genes as being black or white, good or bad, dominant or recessive. Unfortunately, genetics has learned in recent years that this is more a rarity than a rule. In fact, what seems to be more the case is that a given gene acts in a shade of gray. That is to say, a mutated gene will delete something, (like 10% or even less) from a dog's health and appearance. It is precisely here where a good breeder who rigorously tests his dogs before selecting the breeding stock can make a difference. Thus a weak front or rear assembly will not be discovered easily by a breeder who selects for looks alone (even with a partial hip dysplasia a young puppy can move around). However, in a field test weaknesses will show up and we have with this technique discovered, for example, that certain types of pasterns will not allow a Sloughi to sustain several races. Those dogs are consequently excluded from a breeding program.
Unfortunately, the very word genotyping refers to a simple technique in which genes are multiplied to allow them to be tested for something. The same word is used for forensic gene testing to establishing parenthood etc. It is commendable that AKC and UKC are offering this test to help establish parenthood in cases of questionable breeding and to eliminate ultimately irresponsible breeders. In contrast to the genotyping described above, the genotyping for parenthood aims for highly diverse areas of the DNA that do not carry any genetic information. These highly polymorphic regions have the same degree of meaning as fingerprints and can be used, like fingerprints, as unique identifiers for forensic purposes (hence the name DNA fingerprinting). However, like fingerprints, this genotyping does not tell anything about the genetic well being of the individual that has been analyzed.
I am particularly appalled by how some breeders are abusing the public ignorance towards these differences to establish themselves as proper screeners of their breed's genome when in fact they are doing nothing else but verifying parents, something that is utterly unnecessary in a good breeding program. One should stress that DNA fingerprinting is usually done when one suspects the breeder of having registered puppies from a dam and a sire which, in fact, were not the true parents of the puppies in question. In fact, if a customer trusts his/her breeder so little that a genotyping for parenthood is requested, one wonders why a puppy was purchased from that breeder in the first place? While the SFAA has no objection that a member may want to verify the relationship of their Sloughi(s), it is not really necessary as SFAA breeders are ethical and use only temperament, conformation and performance proven Sloughis to further develop the breed. Using knowledge of inheritance of certain characters (for example coat colors), SFAA breeders can predict the outcome of their breeding. Thus, there should and will be few, if any surprises in the SFAA registry.
No genes are identified for the few diseases now known in the Sloughi (a few cases of progressive retinal atrophy have been identified recently in Europe, some cases of myocitic conditions are known in European and American Sloughis, several cases of lymphoma have occurred lately in some closely related Sloughis). Genotyping for health is at the moment useless for the Sloughi. However, SFAA has been in contact with several University based research groups that are working on unraveling the dog genome since 1995. These interactions together with donations of the SFAA to enhance research in the area of dog genetics will provide the necessary information to help with breeding Sloughis free of genetic diseases.
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