Genetic diseases are caused by one or more changes (mutations) on the DNA. They can be monogenic, where one or more mutations are present in a single gene; polygenic, involving mutations of several genes; or the entire chromosome may be defective. Mutations can be hereditary (passed from parents to offspring) or occur spontaneously during embryonic development (de novo mutations).

There are almost 800 hereditary diseases and specific hereditary traits known in dogs until now. Almost half of hereditary diseases and specific hereditary traits are inherited by simple/Mendelian mode of inheritance. Majority of hereditary diseases in dogs are monogenic with an already known mutation. Because of the mostly simple mutations such genetic defects are best studied and consequently well described. Often the discovery of a causal mutation is followed by the development of a genetic test. In polygenic diseases, however, the number of genes involved is so large it is difficult to describe the exact genetic cause of the disease. Due to the complexity of the genetic basis of these diseases they are still not studied enough, and genetic tests are often not available for them.

Genetic diseases can affect all parts of the body. The most studied genetic diseases are diseases of the eyes, skin, muscles, heart, kidneys, neurological disorders, blood diseases, immune diseases, hormonal diseases, metabolic diseases and skeletal diseases. Of all the groups of genetic diseases listed above, genetic eye diseases are the most studied in dogs, as mutations that cause visual impairment in dogs are relatively common. Vision is one of the basic body functions that enables a dog to have a normal quality of life, so the need to research the genetic basis of eye diseases has greatly increased. To date, dozens of mutations are known to be responsible for the development of eye diseases. However, due to the genetic diversity of breeds, some of the described mutations are present only in some dog breeds. This is why one mutation may be responsible for the development of the same specific eye disease in one breed and another mutation in other breeds. To date, several mutations that cause the same eye disease in different breeds of dogs have been described. Therefore, when choosing a genetic test, we must be especially careful to use a genetic test for the mutation that occurs in the selected breed.

In addition to studying genetic diseases responsible for body function defects, a new branch of genetics called pharmacogenetics has been developed. Pharmacogenetics investigates the genetic basis for different responses to drugs between individuals. In dogs, the genetic test for the MDR1 gene mutation (Ivermectin sensitivity) is particularly well-known. Severe neurotoxicosis in animals with MDR1 gene mutation are expressed after application of certain drugs.

Genetic diseases in dogs are untreatable, as gene therapy in dogs is not yet used for general health purposes. Gene therapy is a treatment procedure where an undamaged gene (therapeutic gene) is introduced into the body to replace a defective or missing gene. With the help of gene therapy, we achieve that the mutation or gene damage can no longer cause genetic disease. Due to the incurability of genetic diseases, it is very important to use genetic tests. Based on the genetic test results, in some diseases we can more effectively facilitate the symptoms of the disease or avoid more severe forms of such a disease. Genetic test results are, beside others, an important factor influencing the mating partner selection, as with proper breeding many genetic diseases can be avoided.