Incredible progress has been made in understanding hereditary diseases, which have become recognized as a key clinical issue in small animal practice worldwide. Hereditary diseases are seen both in purebred animals and in mixed-breed dogs and domestic cats, but inbreeding practices certainly favor the more frequent occurrence of specific hereditary defects in specific breeds (eg, anemia caused by PK deficiency in West Highland white terriers and Abyssinian cats but also other breeds). Some are present at birth; others become evident as puppies and kittens grow, and yet others cause disease only in adult animals. Genetic defects can affect any organ system and include malformations, inborn errors of metabolism, and genetic predispositions to infections, immune diseases, drug reactions, cancer, and degenerative diseases.
We do not know the exact number of hereditary diseases affecting dogs and cats because some are poorly defined, and others are caused by different mutations in the same or different genes. Progressive retinal atrophy, for instance, is a very heterogeneous group of blinding disorders based on clinical onset, disease progression, and molecular defects. Similarly, epilepsy is expected to be caused by different gene defects, but little is currently known about the molecular basis. Overall, cats may be affected by a few hundred hereditary diseases and dogs by around 1000.
Most hereditary diseases are known to be inherited by an autosomal recessive trait. This means affected animals are homozygous for a specific gene mutation; an affected animal’s parents appear clinically unaffected but are carriers. Less common are those with an X-chromosomal or dominant mode of inheritance. Predispositions to disease are being recognized to frequently have a complex inheritance pattern: more than one mutation is responsible (polygenic), and disease expression is further influenced by the environment. Examples include hip and elbow dysplasia, cryptorchism, allergies, and cancer predispositions.
The completion of the canine and feline genome sequences and the development of sophisticated and affordable genetic laboratory tools have led to rapid progress in our understanding of health and disease at the molecular genetic level. More than 200 disease-causing mutations have been identified in dogs and ≈3 dozen mutations in cats. These are typically breed specific, but some are ancestral, causing disease in many different breeds. In general, DNA mutation tests for established diseases in specific breeds are the most accurate and precise diagnostic tests available in clinical practice.