A mongrel, mixed-breed dog or mutt, is a dog that is not the result of intentional breeding. Estimates place their numbers at 150 million animals. Although the term "mixed-breed dog" is preferred by some, many mongrels have no purebred ancestors. Furthermore, crossbreed dogs, while literally a mix of breeds, differ from mongrels in being intentionally bred. Although mongrels have at times been considered somehow lesser than intentionally bred dogs, they tend to be less susceptible to genetic health problems associated with dog breeding, and have enthusiasts and defenders who prefer them to intentionally bred dogs.
Although mongrels exhibit great variation, generations of uncontrolled breeding and environmental pressures may tend to shape them toward certain general average body types and characteristics known as landraces, some of which may be developed by people into new breeds such as the Alaskan husky.
At other times, the word "mongrel" has been applied to informally purpose-bred dogs such as curs which were created at least in part from mongrels, especially if the breed is not officially recognized.