Mixed-breed dogs (also called mutts) are made up of a mix of purebred dogs—or often many purebred dogs. And while there are several hundred pedigreed breeds, most domestic dogs are, in fact, mutts. Mutts don't follow strict (or any) breed guidelines. They inherit their unique combination of traits from a range of ancestors, so they can look and act quite differently, even when born into the same litter. However, free-roaming mixed- breed dogs that exist in the same geographic location for generations can have similarities in their general appearance.