Careful selection by mankind throughout the centuries has resulted in genetic diversity within dogs that is unprecedented in any other mammalian species. Although there is still a long way to go before scientists fully understand the complexities of the canine genome, they are well underway. And as scientists come to know more about canine genetics, the results will likely be interesting, important and probably surprising.

Among the many advances in genetics in recent years is the ability to sue certain genetic markers in a dog's DNA for the production of a unique genotype. The method provides absolute identification of individual dogs and parentage verification from one generation to the next. (It does not provide instantaneous identification and therefore does not replace other methods used during kennel inspections.)

All breeds have the same genes, as well as a finite number of gene pairs. Although there is some speculation about the exact number, it is estimated that dogs possess between 50,000 and 100,000 gene pairs in every cell. Variations of these gene pairs, called "alleles" exist for every gene pair. Alleles account for the difference between Chihuahuas and Saint Bernards, and all breeds in between..

A dog's genetic constitution called a "genotype," can be determined by a analysis of a simple bristle swab swirled against the inside cheek portion of a dog's mouth. For example, a possible analysis for a dog taking part in the AKC's Voluntary DNA Certification Program, which tests for 10 gene pairs, might read BD/CE/DD/DZ/EG/CC/GH/CD/CF/AA. Each letter represents one member of a pair of genes, and each gene pair is represented by two capital letters.

For example, "AA" means that on a specific chromosome pair, one chromosome has "A" at a specific site or "locus" and the other member of that pair has an identical "A" allele or gene. Dogs have 78 chromosomes (38 pairs + XX in females and 38 pairs +XY in males) and these are numbered 1 through 38, plus the X and Y chromosomes.

Why test only 10 gene pairs? Because each of the 39 chromosome pairs contains 50,000 to 100,000 gene pairs, it is impossible, prohibitively expensive and unnecessary to test thousands. It has been determined that testing 10 pairs distributed across the 39 chromosome pairs is mathematically sufficient to establish a unique identity.

Among different breeds are different gene frequencies, which means that specific alleles occur more or less frequently than others. One goal of the Parent Breed Club Program is to understand these using 17 markers and learn how to efficiently structure tests for parentage across different breeds. These results will permit the AKC to establish a program based on extensive knowledge of the genetic markers used across all breeds. No one has ever collected and analyzed this amount and kind of genetic variation in purebred dogs. The results will undoubtedly provide useful information to the fancy and to the scientific community.

How does this relate to parentage verifications? In every generation, there is a "shuffling" of genetic information. This shuffling is called "genetic recombination," and its consequences for breeders are extraordinary. For example, 50 percent of a dog's genes come from its sire and 50 percent from its dam, which means that 50 percent of a sire or dam's genes are not present in their offspring. It's no wonder that breeding great dogs takes understanding, dedication and patience. 50 percent of what you hope to see from any great sire or any great dam is discarded every time a new pup is born.

The shuffling of genetic information is responsible for both the successes and the failures as new pups are whelped. Without genetic variation, no new combinations would result and the genetic challenge of producing better dogs--by carefully selecting and breeding the best to the best--in succeeding generations would not be a reality.

What does all this mean for breeders? It means that we can now begin traveling down a long road--a road that begins with the absolute identification of each dog by DNA. and continues with foolproof parentage verification. It follows that the AKC registry will have absolute integrity. That fact closes the doors to possible questions about the registry, and permits fanciers to focus on what the sport is really all about--breeding better dogs.

by Dr. James Edwards

Dr. Edwards  is the AKC Compliance Operations and DNA Educational Services Director.
For additional information on the AKC's DNA Certification Program, go to the AKC website:

What is a gene? The gene is the basic unit of heredity. Each gene, acting alone or with other genes, determines one or more canine characteristic. All of the genes that constitute the hereditary makeup of an organism are called the genome. Genes occur in strands of genetic material within the cell called chromosomes. A dog is composed of a large number of cells that are genetically identical. The first cell of a particular dog formed when egg and sperm united contains one set of chromosomes form each parent. The canine genome is made up of 39 pairs of chromosomes (one set form each parent) that contain approximately 3 billion base pairs of DNA, or about 100,000 or so genes. Each gene generally occupies a particular position within a particular chromosome.

What is DNA? The chromosome is made up of two very long single strands of a chemical called DNA (deoxyribonucleic acid) that are wrapped around each other to form a double helix. This DNA never leaves the nucleus of the cell--it is like a reference library that contains the genes (functional) regions) that determine how the organism will develop. DNA strands are made up of 4 basic units linked together in pairs. The entire canine genome contains about 3 billion base pairs. A small gene may contain 100 of these base pairs; a larger one may contain 10,000 base pairs.

What is a genetic marker? Scattered throughout the chromosome are short repeated groups of these base pairs known as micro-satellites, or markers, that can be used to track defective genes. Hundreds of these distinctive sequences have been isolated along the canine genome for use in mapping genes. To find a marker that is "linked" to a disease, researchers may examine hundreds of markers from animals with and without the disease before they find one that is located so close to a disease gene that it is almost always inherited along with the disease caused by that gene. The closer the marker is to the disease gene itself, the more accurate the test. Finding such a marker narrows down where to look for the disease-causing gene, which could ultimately lead to a more specific test for the gene itself. 

What is a mutation? A mutation is a genetic mistake that scrambles the instructions given by a gene. Mutations may be good, bad, or indifferent. In the case of renal dysplasia, it is believed the presence of mutations in one or perhaps two different genes causes the glomerule of the kidney to stop developing.

What are dominant and recessive genes? A dominant gene will express itself when the puppy inherits only one copy of the gene (from sire or dam). If a disease-causing gene is recessive, a dog with the gene can be bred to a dog without it and will not produce the disease, although it will produce carriers of the gene. If the gene is dormant, both parents must be free of it to avoid producing affected puppies. Again, more than one defective gene may be needed to produce a disease.

What is the two gene theory of renal dysplasia inheritance? Researchers believe that renal dysplasia is probably caused by the presence of two defective genes rather than one (although it may be cause by a single dominant gene that is affected by other factors--i.e. has incomplete penetrance.) The two-gene theory, with one of the genes being dominant and the other recessive, would explain many of the mysteries of RD. Imagine that RD is a door with two locks, the regular door lock and a deadbolt lock. In order to open the door, you will need two keys. But you need the right two keys. Two door keys or two deadbolt keys will not do it. If you mate a dog with RD1 to a bitch with RD1, the puppies will have normal kidneys. If your dogs are highly line bred, and therefore highly homozygous for RD1, you will not have a problem until you outcross to a line with RD2. This would explain how a breeder can go along with healthy dogs for many years and suddenly have a whole litter with renal dysplasia. It would also explain the rare case in which two dogs were biopsied clear produced affected puppies. Even if two defective genes are involved, we only need to identify and eliminate one of these genes to avoid producing any affected Shih Tzu. The new VetGen linked marker test for one of these defective genes is a major step in that direction.

What about the genetics of JRD? Shouldn't the actual underlying genetic situation be known before breeders act? The underlying genetics of JRD is not known at present, and may not be known for some time. Theories include:

1) That abnormalities in two genes, one dominant and one recessive, must simultaneously be present.
2) That a common dominant gene with incomplete penetrance is responsible, but it is affected by other factors (called incomplete penetrance).
3. That an even more common recessive gene, again with incomplete penetrance is responsible.

Any of these theories could be correct because once you invoke incomplete penetrance, differentiation between theories becomes difficult. However, for purposes of using this test, the underlying genetic mechanism is not relevant. 
What you have is a marker that when present indicates a greatly increased risk, and when absent, indicates a greatly reduced risk. It does not matter at this point whether it is predicting a mutation in one of two required genes, a mutation in a dominant gene, or a mutation in a recessive gene. Its applicability is exactly the same.

If we reduce the M allele frequency significantly, would we have a significant negative impact on genetic diversity in any of the breeds? A gradual reduction of an allele that has about a 30% frequency is not a risk to diversity. Also it is unlikely that all breeders will embrace the test simultaneously. Some breeders will eliminate or greatly reduce the M marker; others will do this to some extent, while others will ignore it. This mix of responses will minimize any impact. Keep in mind that when the genetic defect is found, it will probably have a relatively high frequency, putting the breed in exactly the same situation.

What about the cost of the test? Why should as marker test that is 80% predictive cost as much as a direct test of the mutation that is 100% predictive? The cost is not based on a differential value to the breeder, but on the costs of providing the service and the cost of research to develop the information to develop the test. VetGen has yet to make a profit on any test it has offered. 

For individual tests contact VetGen  or 1-800-483-8436, or go to the VetGen website  for an order form and more information. 

Every DNA sample sent directly to VetGen is coded upon receipt, making references to specific dog's identity and genetic status entirely confidential between VetGen and the owner of the dog.

For additional information go to the ASTC website