A mixed-breed dog (also called a bastard, mutt (shortened from muttonhead), mongrel, tyke, mixed mutt, cur, or random-bred dog), is a dog that is a mixture of two or more breeds, or a descendant of feral or pariah dog populations. Except for extreme variations in size, dogs interbreed freely, mixed-breed dogs vary in size, shape, and color, making them hard to classify physically. Historically, all purebred dogs have been selected from a mixed-breed population. See Golden Retriever for an example.
All possible body shapes, tongue color, ear types, and tail styles can appear in mixed breeds. Extremes in appearance, such as the flattened face of the English Bulldog or the extremely curled tail of the Pug, seldom survive even the first crossbreeding. Mixed breeds also tend to have a size between that of their parents, thus tending eventually toward the norm.
It's important to note that all dog breeds are man-made creations. Dogs were traditionally bred for specific functions. Most existing dog breeds began as mixed breeds, either by random occurrence or by deliberate crosses of existing breeds.