Genetics are very important to those who breed service dogs. While it is true that it is possible to purchase and train your own dog to become a service dog, the vast majority of these never go on to be fully functioning service dogs. Organizations, such as Canine Companions specifically breed their own dogs and look for particular traits like longevity and weed out genetic medical conditions like hip dysplasia. Their research team also does many different tests on their puppies to judge their problem solving capabilities and temperament. You can find out more about what they are doing at the section of the Canine Companions research page.
