Besides physical characteristics such as coat and eye color, diseases can also be inherited. They, too, are controlled by dominant and recessive genes. A disease or defect caused by a dominant gene is obvious, because the dogs that carry the gene develop that particular disease. When this happens, responsible breeders know not to repeat that breeding. A disease caused by a recessive gene is a little trickier to deal with. A disease-causing recessive gene can be passed on for generations without causing problems, but if it becomes paired with another recessive gene carrying the same disease, then the disease expresses itself, to the consternation of the breeder. Some diseases now have genetic tests to identify carriers of recessive genes, and researchers are working to develop more.

Besides helping breeders to identify carriers, genetic tests allow them to breed away from defective genes and ensure that defective genes aren't reintroduced in future breedings. For instance, von Willebrand's disease, a recessive bleeding disorder, is a problem in Doberman pinschers. Thanks to a genetic test and screening program, Doberman pinscher breeders can identify dogs that carry the disease or are affected by it. Using this knowledge, they can decrease the frequency of the defective gene by selecting normal-testing dogs for breeding. They can also breed carriers to normal-testing dogs and then use the normal-testing offspring in their breeding programs. That allows them to preserve the good qualities of the carriers without eliminating them from the gene pool, which is important for genetic diversity.