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Genetic Faults...Article: Genetic FaultsAuthor: MindiBown Kennels Selecting for the elimination of genetic faults falls into three broad categories. 99% of faults in the purebred dog are caused by either: 1. Dominant genes 2. Recessive genes 3. Polygenitic genes Dominant Faults caused by a dominant gene are a non issue. To the best of my knowledge there are none. Well, none that is common anyway. Quite simply, If a dominant gene caused the fault, it should already have been eliminated. The reason for this is simple. In the case of dominant, if a dog carries the gene, it also exhibits the problem. The answer is simplicity itself. Don't breed from that dog. The fault is removed from that line in a single generation. Recessive Faults caused by recessive genes are slightly more difficult to remove. The reason for this is because the dog may CARRY the gene that causes the problem but does not actually suffer from the problem itself. Let's take PRA (Progressive Retinal Atrophy) as an example. This is not a problem found in the Chihuahua but I will use it as an example of a typical recessive problem. Because a recessive gene causes PRA, a dog that actually suffers from the affliction MUST by the nature of genetics carry two copies of the gene. ALL offspring of that dog therefore WILL inherit one copy of the appropriate gene. If a dog shows clinical signs of PRA, it must be removed from your breeding program. (I say that with some reservation, as there may well be a circumstance where you WOULD breed this dog. For the purpose of this article however I will take the simple approach. I will cover why breeders will sometimes knowingly breed from a dog with a fault in another article) Unfortunately the problem doesn't end there. It is quite possible for a dog to carry a single copy of the PRA gene and it will NOT actually develop PRA itself. No amount of clinical testing will ever show you that PRA exists, because it doesn't. The dog is healthy. However, put two such dogs together in a breeding and a percentage of the offspring will inherit a copy from both Mum and Dad. These pups will develop PRA as they get older. They now have two copies of the gene. A percentage of the offspring will inherit one copy from either Mum or Dad. These pups will never develop PRA themselves but WILL carry it to the next generation. For this reason, breeders commonly refer to such dogs as carriers. Of course, a percentage will also inherit nothing and will neither exhibit the problem nor carry it. They will be healthy dogs and will produce healthy dogs. The only known way of combating a recessive problem is in an understanding of how it flows from one generation to another, and an accurate pedigree system so that you know both the ancestors and progeny of any given animal. (I'm ignoring DNA tests as a possibility as they are available in relatively few cases) There are a few things that you will know for sure that will help in combating this type of problem. Firstly, if a dog in your kennel develops PRA, you know positively that BOTH parents are carriers. I've often heard breeders say that a problem known to be caused by a recessive gene was caused by a certain line of dogs. “ I've never had that in my kennel until I used xyz stud. Obviously he caused it” Wrong! For a problem such as PRA to develop, the pup has to get a copy of the appropriate gene from BOTH parents. Your bitch may have never produced the problem before, but that merely means she's never used the wrong stud before. This knowledge is really your best weapon. How? Because now you can start tracking back and forward in the pedigrees. Tracking Back Quite frankly, tracking back in a pedigree is a useless exercise unless you are in one of those rare situations where it is known POSITIVELY which dog brought the problem into the country or area. This does occasionally occur and if you find yourself in this situation, you can draw a line from your dog directly to the known introducer. From this you know that every single dog in this line IS a carrier. Furthermore, you also know that every pup ever produced by any of these dogs COULD also be a carrier. Tracking Forward Tracking forward in your pedigree means knowing that every single pup produced by EITHER parent COULD also be a carrier. From that point on, it's decision time. If the problem you're dealing with is serious, and you're determined to eliminate it at all costs, you would remove ALL of the suspected carriers from your breeding. If doing this eliminates a big chunk of your breed, that's probably not a smart move. More commonly, the knowledge is filed away to be used as one more consideration when planning a breeding. When the time comes to decide which bitch goes to what dog, you have to ask yourself, “What might I possibly GAIN from this breeding?” Good tail set, improved ears and a more assertive temperament. What might I possibly LOSE from this breeding? Too much height, and a PRA carrier or sufferer. It's YOUR call. You're the breeder. Some examples of just some of the problems believed to be caused by a recessive gene: You'll be pleased to know that none of the above problems (excepting cleft pallet) are common in Chihuahuas Polygenitic If you intend to be a dog breeder, Polygenitic is a good word to learn. Quite apart from the usefulness of knowing it's meaning, you can have a lot of fun with it. Some day you'll be in a group of breeders and one will start to try to impress the others with their knowledge of genetics. They'll throw words like dominant and recessive around like confetti. All you have to do is make a simple comment with the word Polygenitic in it……then sit back and time how long it takes the other breeder to change the subject. The fastest I've seen is 2.5 seconds. The word Polygenitic actually describes a trait rather than a gene. The best way to explain this trait is to go back to recessive/dominant again. In simple genetics a feature is either on or off. There or not there. A dog is either black or it isn't etc. A dog has PRA or it doesn't. You don't see a mild case of PRA in one dog and a more severe case in another. That's the nature of dominant/ recessive. But some things aren't just black and white. There can be a range of grey in between. Anything that can occur in degrees is usually termed a Polygenitic trait, or is said to be of a Polygenitic nature. If anyone asks you for an example, say “Size” Think of a problem that comes in degrees of seriousness. There's a good chance it is Polygenitic in nature. Within our breed, the example I will use is the scourge of the Toy Dog group, Luxating patellas. Another example I could have used is Hip Dysplasia. These are afflictions that can range from very mild to quite severe. A typical Polygenitic trait. Now, I'm going to ask you to stay with me. This concept is actually easy to understand once someone has explained it to you. The difficulty is in the explaining. Once you understand, try to explain it to someone else. You'll see what I mean. When dealing with problems such as luxating patellas, you must follow a set pattern. Step 1. You must have a way of grading the problem from no problem through to severe problem. Within reason, you should have as many gradients as you can handle. The grading system we use in our kennels for patellas has 5 levels, from 0 to 4. Make sure that what constitutes each individual grading is very clear in your mind. Step 2. Come up with a maximum grading that YOU believe is the most you will accept. That's the hard part! Let's look at what you DON'T want to do. 1. Select a level that is too permissive. Let's say you chose “I won't breed with any Chihuahua that is physically unable to walk” Big deal! You'll eventually reduce the problem somewhat but can you live that long? 2. Select a level that is too restrictive. Let's say you decide, “I'll only breed Chihuahuas with perfect patellas” With the current level of the problem in our breed, you've just eliminated 90% of all the Chihuahuas in Australia, including a lot of the best ones. What you will end up with if you breed only from the remaining 10% is a bunch of butt ugly Chihuahuas with very strong knee caps. Here's what you're aiming to do. Eliminate the worst cases and breed only from the rest. What level that is, depends on how bad your dogs are now. Let's say you have a really bad problem right now. You may chose to only eliminate the grade fours. (Actually, if your problem were that severe I would suggest you find a bunch of good pet homes and start again. But let's go on just to illustrate the point.) Having eliminated all the grade fours you would then breed only from grade 3’s or less. But surely you must have at least one dog that is a grade 1 or 2. Think about putting those two dogs together. (Assuming there is no other reason for not doing so, such as they are brother and sister.) Or perhaps you can look outside your kennels for a sound stud dog. What you are coming up with is a total score of all four patellas between 2 dogs. If your worse dog has a grade 3 on each patella, and your best Stud has 2 grade 1’s, your total score is 4+ 4+1+1= 10. As I said earlier, if that really is the best you can do, I'd start again. But if that weren't possible you'd have no choice. Do it! The progeny of this breeding should come out with a slightly reduced problem. Let's say the next generation allows you to put a bitch with a 4 and a 3 with a stud with two 1’s. You've made an improvement haven't you? If you keep this up you will eventually get where we are now. Our maximum permissible score is now 3. You started your program at 4, and you are doing the figures now, to see if you can reduce the maximum score to 2. So your just waiting to have some updated gradings done and you can see where you stand. A couple of points on your grading system: 1. Be totally clear on what constitutes each level. 2. . Be consistent in that grading. In the case of patellas, it's best to have it done by a vet. Make sure you use the SAME vet each time so there is a consistency in the grading. We've just changed vets so all our dogs are being re-graded by our new one. There was nothing wrong with our old grading except that it was done by someone else. We need to be sure that if a new dog is graded at 0,0 by our new vet, that we can comfortably assume it be the same as a 0,0, from our old vet. 3. . Remember that the goal is constant and steady improvement, not to fix the problem overnight. 4. . And lastly. It's no good fixing one problem if in the process you have allowed another one to get a stronger toehold in your kennels. In other words. Don't just attack patellas and ignore all the other aspects that you are aiming for in your breed. Not Just Patellas This method can be used on any problem that occurs in degrees. We have just turned our attention to the soft palette problem in our breed. From what research we have been able to do we can find no positive source that would indicate that this is an inherited trait and if so, by what degree. We are going to have to assume that it is Polygenitic in nature purely due to a lack of feasible alternatives. Our first step is to totally understand precisely what is happening inside the throats of our dogs. This we have accomplished. The next step is to insure there are no other possible explanations that may cause the same type of breathing difficulty. Then, and only then we can try to come up with a grading procedure. This won't be as easy as patellas and initial conversations with our vets have not proved promising. We now have two separate vets thinking about our request and trying to come up with a solution. It may prove difficult to do accurately but in this case I think we have to adopt the policy that a poor grading system is better than no grading system. Once that is accomplished, we will do precisely what I've described above with selecting our worst and our best and choosing an acceptable level to start. (C)Copyright 2007, this article is copyright protected Special thanks to MindiBown Kennels for permission to display this article on RoyaltyChi Chihuahuas website |