There is increasing evidence that many forms of congenital and acquired disease in veterinary medicine are of a familial origin. As of 2005, 430 canine diseases and 180 feline diseases have been identified as familial. It has been estimated that 5–10 new familial diseases are identified every year. The significant number of familial veterinary diseases has been thought to be associated with the improved control of diseases caused by environmental factors as well as the desire to breed animals to maintain an appearance and selecting animals from a small group of popular founders (founder effect). It is reasonable to consider a familial etiology for common feline and canine heart diseases, particularly those with strong breed predispositions.

Many forms of congenital and acquired disease have a strong breed predisposition, suggesting a familial etiology. The small animal practitioner may be asked to provide consultation on genetic issues in order to provide information to owners of breeding animals to aid them in breeding decisions and to provide pet owners information about etiologies of disease.

Breed-specific lists of known and presumed inherited diseases in the dog and cat can be found at the following websites:

 Cats: www.fabcats.org/breeders/inherited_ disorders/index.php

 Dogs: www.vet.cam.ac.uk/idid and http://www.upei.ca/~cidd/intro.htm (VIN editor: The second link could not be accessed as of 2-17-17.)

Even though the number of genetic diseases in small animals is significant, the actual genetic cause of it has not been commonly known. Even in the absence of knowledge of a molecular cause, a great deal of information can be provided just from an understanding of pattern of inheritance.

A Few Key Points Should Be Remembered

Genetic diseases are most commonly observed in pure-breed animals. Breed organizations have intentionally limited their gene pool by preventing breeding to other breeds. Therefore, the gene pool is of a limited, closed size and aggressive removal of breeding animals because of the identification of a defect will make the gene pool even smaller. One must carefully recommend removal of breeding animals based on severity of defect, mode of inheritance, and importance of a particular animal to the breed.

Genetic mutations rarely have an all or nothing. For some diseases, a genetic cause has now been identified, and a genetic test can be done. The presence of the mutation does not mean that all animals will show the trait, or the same severity of the trait; this depends on disease "penetrance," a poorly understood phenomenon that likely involves genetic modifiers.

Identification of Mode of Inheritance

Knowledge of the mode of inheritance of a specific trait can be used to provide guidance about reducing the prevalence of a trait within a particular line of animals. The determination of the pattern of inheritance will be useful for determining the best recommendations for breeders and will allow the development of plans to exclude or include affected animals in breeding programs. The most common include autosomal recessive, autosomal dominant, X-linked recessive and X-linked dominant.

X-Linked

X-linked traits are almost always recessive and should have the following criteria: more affected males than females; an affected male crossed with a normal (non-carrier) female should produce silent carriers; silent carrier females should have a 50:50 chance of passing the trait on to male offspring; and affected females are the result of a cross between a silent carrier female and an affected male (not a frequent occurrence).

 Example: Progressive retinal atrophy in the Akita

 Recommendation: If animals with an X-linked trait are used for breeding, they should always be bred to an unrelated line; this will decrease the presence of the trait in the line.

Autosomal Recessive

Autosomal recessive traits should have the following criteria: the disease should appear to "skip" a generation (parents do not usually show the trait); males and females should equally show the trait; the mating of two silent carriers (heterozygotes, carry one copy of the abnormal gene) should produce offspring that show the trait in a 3:1 ratio; and if both parents show the trait, all offspring should show the trait.

 Example: Diabetes mellitus in Keeshond

 Recommendation: If animals with an autosomal recessive trait are used for breeding, they should always be bred to an unrelated line; this will decrease the presence of the trait in the line.

Autosomal Dominant

Autosomal dominant traits should have the following criteria: males and females should equally show the trait; every affected individual should have at least one affected parent; and all heterozygotes (one copy of the abnormal gene) are affected and transmit a mutant gene to half of their offspring. Affected animals may carry the genetic mutation on one or both copies of the gene. If it is on both copies of the gene (one inherited from each parent), they are considered homozygous for the mutation and will pass one copy of the mutation to all of their offspring.

 Example: Dilated cardiomyopathy in Doberman pinscher

 Recommendation: If animals with an autosomal dominant trait are used for breeding, they will have a 50–100% chance of passing on the trait (depending on if they are heterozygous or homozygous for the trait). Therefore, only those with the most mild form of the disease and the most positive attributes should be selected to be used.

Utilization of Molecular Information for Screening and Therapeutic Issues

In some cases, a molecular genetic cause for a specific trait has been identified and a genetic test is available. Genetic tests are generally a PCR test that identifies either a marker for the disease or identifies the actual genetic mutation. PCR is a method that takes a small amount of DNA provided by the clinician or owner and amplifies a region of interest so it can be carefully inspected. DNA can be usually provided in a blood sample in an EDTA tube, a buccal swab, or even a semen sample. The DNA will be inspected for the abnormality by the lab, and the presence or absence of the marker or mutation identified. However, breeders and owners should be cautioned and advised how to best use the information. The results should be carefully considered and should be weighed against the severity of the trait, the size of the breed's gene pool, the mode of inheritance of the trait, and the positive traits that this individual animal brings to a breed. In some cases, strict screening and removal programs may be very detrimental to small gene pools in specific breeds; breeding recommendations should be carefully designed.