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The fact that a pet is not a purebred does not mean that genetic diseases shouldn't be screened for. Diseases caused by recessive genes are less likely to be problems in mixed‐breeds but many breeds share susceptibility to the same genetic defects, especially disorders with more complex inheritance that just a single gene mutation. Statistically, purebred dogs are not always less healthy than mixes and dominant genes can affect offspring regardless of how far from purebred they are (see link to video titled Mixed breed dogs have genetic risk too: www.youtube.com/watch?v=Ce1u_ZqwqMo).

We can sometimes infer susceptibility to genetic diseases just by looking at size or phenotype. We might expect any large or giant breed to have greater risk for hemangiosarcoma and hip dysplasia than a much smaller dog, although the risks are actually based on family history.

Gene defects may cause a direct problem every time, or they may be turned on or off by factors such as the pet's environment, diet, infection, medications or surgery. For example, a cat may have a genetic susceptibility to diabetes mellitus but may only develop the disease if it becomes overweight. A collie may have the MDR1 genetic defect that causes it to get sick from the drug ivermectin but if it never received that or other drugs implicated with this variant, the problem wouldn't be evident.

Fortunately, DNA screening can now tell us what breeds are likely in a pet's heritage as well as allowing us to test or screen for individual genetic diseases. Panels of DNA tests are available as well, so we can test for dozens or hundreds of DNA defects with a single sample of blood, saliva or cheek cells (see 3.6 Genetic Testing). These test panels are inexpensive enough to be used widely as screening tools. Unlike other types of testing, such as thyroid screening, a DNA panel typically only needs to be done once in a pet's life.

Some care must be taken when selecting a laboratory for DNA testing. Manufacturers are not required to disclose their testing methodology nor is there any oversight of the laboratory running the test. Direct‐to‐consumer DNA tests are popular, but the results may be suspect. The WSAVA's Canine and Feline Hereditary Disease website (https://wsava.org/committees/hereditary‐disease‐committee/#:~:text=The%20World%20Small%20Animal%20Veterinary,now%20and%20in%20future%20generations) lists available DNA tests by breed, disease and testing laboratory, and provides links to peer‐reviewed citations. Organizations such as the Orthopedic Foundation for Animals (www.ofa.org) and the International Partnership for Dogs (www.dogwellnet.com) can provide advice on available testing. University‐run laboratories are usually highly qualified, though they generally run only individual tests versus panels.

Not every genetic problem is caused by a single gene defect (see 3.3 The Genetics of Disease). Some, such as hip dysplasia, involve multiple genes and a wide range of possible presentations. Instead of a single test, radiographs for hip dysplasia look for the phenotypic evidence, not the genetic defects themselves. Some screening tests for genetic disorders, like ophthalmologist or cardiologist evaluations, or thyroid screening, must be periodically repeated (see 3.4 Predicting and Eliminating Disease Traits).

Genetic testing is especially important for anyone thinking about purchasing a purebred dog or cat, or breeding purebred or mixed‐breed pets (see 3.10 Advising Clients on Selecting an Appropriate Pet). It is the responsibility of everyone who breeds animals to do so carefully and with a good understanding of the genetic risks for their breed or breeds (see 3.11 Integrating Genotypic and Phenotypic Testing). Genetic testing before breeding should be considered a standard of care today. No one should be breeding an animal without screening for genetic diseases. No one should be buying a purebred pet whose parents have not been screened. Many breed associations and clubs have genetic screening information on their websites and some offer screening clinics for common diseases present in their breed.

For the average client with a pet, we are focusing on problems that pet might develop in the future. For breeding, we also have to focus on problems that pet could pass along to its offspring. The testing then becomes more indepth.