A mongrel, mixed-breed dog, or mutt is a dog that does not belong to one officially recognized breed and is not the result of intentional breeding. Estimates place their numbers at 150 million animals worldwide. Although the term "mixed-breed dog" is preferred by some, many mongrels have no known purebred ancestors. Technically, crossbreed dogs, and "designer dogs" while also a mix of breeds, differ from mongrels in being intentionally bred. Although mongrels are viewed as of less commercial value than intentionally bred dogs, they are thought to be less susceptible to genetic health problems associated with inbreeding (based on the theory of heterosis), and have enthusiasts and defenders who prefer them to intentionally bred dogs.