Genetic tests vary on what they are able to identify, and therefore how they can be used in managing genetic disease. Some tests measure the phenotype, or what can be seen in the animal. This may not directly relate to the genotype, or the genes regulating the defect.
Screening for cataracts, ausculting for heart murmurs, thyroid autoantibodies, hip and elbow radiographs, urinalysis for crystals or metabolites, skin biopsies, and observation of behavioral traits are all tests of the phenotype. Most tests of the phenotype only identify affected individuals, and not carriers.
Direct gene tests utilizing PCR or polymerase chain reaction, are a direct measurement of the genotype. These can be run at any age with a blood sample or cheek swab, regardless of the age of onset of the disorder. With tests of the genotype, you can identify affected, carrier, and genetically normal animals.
Some defective genes can be linked to a genetic marker, which can be tested for. Linked-marker based tests do not identify the defective gene, but a marker that lies close on the chromosome.
During the formation of egg or sperm, a crossover and trading of genes occurs between paired chromosomes. If a crossover between the marker and the gene occurs, the marker will no longer be linked to the defective gene. False positive and false negative results will occur in the individual, and in all of its descendants.
Due to this phenomenon, linkage test results must be compared with results from other family members to determine whether they correlate with the known genotype of relatives. Linked marker tests include those for cerebellar ataxia in Italian Spinone and primary hyperparathyroidism in the Keeshond.
Most tests of the genotype are for genes that are the sole and direct cause of a disease or condition. Others test for susceptibility genes that provide liability for the disease. Some of these susceptibility genes are necessary for the animal to be affected, though other yet undiscovered genetic or environmental factors are also necessary.
Examples are cord1 PRA in English Springer Spaniels and Miniature Dachshunds, and degenerative myelopathy in several dog breeds.
Other susceptibility genes are found to occur at a greater frequency in affected animals, but are not present in all affected animals. An example is the susceptibility gene for perianal fistula/anal furunculosis in German Shepherd Dogs.
Many owners and breeders ask what tests should be done in their cats and dogs. The answer depends on whether the cat or dog is going to be a pet, or be used for breeding.
For a pet, it is only important to know that it is not going to be affected by a health-related disorder. For breeding animals, it is important to know if they carry disease liability genes that they can pass on to their offspring. Further information is available in the chapter on Genetic Counseling.