SIMPLE GENETICS
Why Health Testing Is Important


Genetics, the way things are passed from one generation to the next, is very complex. This is an extremely simplified explanation to help you understand the importance of health testing, even if your dog does not have any known health problems.

For every feature of your animal (nose color, coat color, shape of the eye, length of the legs, coat length and density, height, etc.), it has received 1 gene from each parent, so it has 2 genes for each feature. Some genes are dominant and some are recessive. Dominant genes are generally those that are expressed (for instance, they would generally determine the coat color, eye color, etc.). Recessive genes are usually not seen if they are paired with a dominant gene (in most cases, black is dominant over cream, so if a dog had a gene for black from one parent and a gene for cream from the other parent, the dog would appear to be black) but would be seen if paired with another recessive (if the dog inherited a cream gene from each parent, it would look cream).

Dominant genes are usually indicated using a capital letter and recessive genes with a lower case letter.

If we were to consider breeding a black dog with 2 black genes (BB) to a cream dog (cc), the puppies would get a B from 1 parent and a c from the other parent, so all the puppies would be black (they would all have be Bc, and black would be dominant over cream).

If we then bred one of the puppies (Bc) to a cream (cc), some of the puppies would get a B from the first parent and all would get a c from the other (Bc) and would be black. Some others would get a c from the first parent and a c from the other parent (cc) and would be cream. Statistically, 25% of the puppies would be black and 75% would be cream.

If you bred a black Bc to another black Bc, some of the puppies would get a B from the 1st parent and a B from the 2nd parent (BB) and would be black (they would not carry a gene for cream and so could not produce any cream puppies). Some would get a B from 1 parent and a c from the 2nd parent (Bc) and be black but would carry a gene for cream (and could produce cream if bred to a Bc or cc). Some would get a c from each parent and be cream (cc). Statistically, 25% would be BB, 50% would be Bc, and 25% would be cc.

This is important if we are trying to breed healthy dogs because many genetic diseases are inherited as a simple recessive like the coat color example. Progressive Retinal Atrophy (PRA) is one of these diseases, which causes blindness. If we use “C” to indicate a clear gene and “c” to indicate a carrier, we can see how a dog that does not have PRA can produce puppies with PRA.

A dog that is CC will not have PRA. A dog that is Cc will not have PRA but will carry the gene for it, and a dog that is cc will have PRA. If a CC dog is bred to a CC dog, then none of the puppies will have PRA and none will carry the gene, because both parents will pas a C to the puppies. If a CC dog is bred to a Cc dog, the all will get a C from the 1st parent. Some will get a C from the 2nd parent (and be CC—no PRA and not a carrier) and some will get a c from the 2nd parent (and be Cc—no PRA but will be a carrier). If a Cc is bred to a Cc (none have PRA but both carry the gene), some puppies will get C’s from both parents (CC—no PRA and not a carrier), some will be Cc (no PRA but carry the gene) and some will get a c from both parents (and be cc—they will have PRA).

The American Kennel Club (through the Canine Health Foundation), breed clubs, and individuals have all donated hundreds of thousands of dollars into genetic research to help eliminate producing affected dogs. In the case of PRA, the gene or a marker has been found for many breeds. The DNA test to determine the status of a dog (clear, carrier, or affected) is $240 (plus the veterinarian’s fee to draw blood plus Fed-Ex shipping) at this time (see www.optigen.com and www.vetgen.com to see what DNA tests are currently available—research is continuing on many other diseases).

With a DNA test, it is possible to not produce an affected dog. There is nothing wrong with producing carriers, as they will never have the disease. Carriers (Cc) should only be bred to clears (CC) to keep from producing PRA.

With other diseases, DNA tests are not available, and breeders must do a lot of research to keep from producing affected dogs. Without a test, though, it is still a guess, but based on knowledge. This is why it is important to register results of all health tests with the Orthopedic Foundation for Animals.

If we had a breed that is known to have Addison’s Disease, for example, we would research dogs related to the dog we are interested in breeding to, as well as our own dog. Has the dog ever produced Addison’s Disease (if so, it is at least a carrier)? Do any of its littermates have the disease (if so, their parents were at least carriers and there is a chance that the dog you are interested in may be a carrier)? Have any of its littermates or siblings from other litters produced the disease? By doing research, it is possible to reduce the chances of producing the disease, but it is also possible that 2 carriers of the disease were never bred together and they may be carriers of the disease.

This is further complicated by the fact that not all diseases are inherited as simple recessives as we’ve described. Some are sex-linked (only carried by one sex but passed on from generation to generation), some are polygenic (require the combination of several genes to produce the disease—hip dysplasia is one of these, and the reason that 2 dogs with excellent ratings can produce a puppy with dysplasia). Some diseases are late onset (don’t show up in the parent until the dog is 3-10 years old). There is also incomplete dominance. Genetic mutations can also occur. And a further complication is that some disease can be affected by the environment. Hip dysplasia, for example, can be caused by diet, the type and amount of exercise, etc. in a dog that would not otherwise have had the disease.

These genes are also passed to mixed breed puppies (including the “Designer Dogs”). The genes don’t go away. While the offspring of a Labrador Retriever with a Poodle (called a Labradoodle) may not exhibit any diseases, when/if they are bred the puppies may inherit diseases since the parents may have been carriers without exhibiting the disease.

Most breeders will be honest with their puppy buyers and will provide contracts with guarantees. The guarantee will tell the puppy buyer what the breeder will do if the puppy develops a genetic disease (and what the puppy buyer must do to keep the guarantee—jogging a 4-month-old puppy on pavement a mile a day would probably produce hip dysplasia without the gene). This is why it takes many years to develop the knowledge about a breed to do the best to produce healthy puppies.



North Carolina Responsible Animal Owners Alliance

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