A mongrel, mutt or mixed-breed dog is a dog that does not belong to one officially recognized breed and including those that are the result of intentional breeding. Although the term mixed-breed dog is sometimes preferred, many mongrels have no known purebred ancestors. Crossbreed dogs, and "designer dogs", while also a mix of breeds, differ from mongrels in being intentionally bred. 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.

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.

Estimates place the prevalence of mongrels at 150 million animals worldwide.

 

Note:   The above text is excerpted from the Wikipedia article Mongrel, which has been released under the GNU Free Documentation License.