Scientific research has shown how DNA variations are linked to particular syndromes. By analysing the differences between the DNA of healthy animals and in animals that are carriers or sufferers of certain hereditary diseases, we can identify in which gene and at what position a mutation in the DNA has occurred which causes the hereditary disease. By comparing the symptoms of a disease with variation in the DNA, a diagnostic test will prove its efficacy and publication in a scientific article generally follows. Often these articles focus on just a single breed.
The availability of DNA tests to establish whether an animal is a carrier or affected of a hereditary disease enables breeders to use this knowledge when selecting a mating partner for their breeding animals. By doing so they can prevent offspring being born that are affected by hereditary diseases and can improve animal welfare.
If the result of a DNA test shows that an animal is an affected (two copies of a mutation), normally speaking one assumes that this animal will develop the hereditary disease. If the result of a DNA test shows that an animal is not a carrier of the mutation which causes the particular disease, this is no guarantee that this animal will never develop this hereditary disease, as mutations can of course occur at other places in the DNA which cause the same syndrome.
Mutations that have been described and validated in a single breed can also appear in other breeds, but these are often no longer published in scientific articles. Laboratories performing these tests can establish whether these mutations can be prevented in other breeds. It is not easy to determine how reliable a particular test is for a particular breed.
This basis described above applies in general to hereditary diseases. Naturally, inheritance and illnesses are biological processes, and so exceptions can occur. The specific information in a test describes the observations that may possibly apply.