Health
Breeding and over-breeding pure-bred dogs with other pure-bred dogs of the same breed has allowed some hereditary health conditions to continue and expand. A major one is Hip Dysplasia, which could have been bred out long ago. Dogs from puppy mills frequently have more health issues than others, because the breeder is only interested in their bottom line. Since a Mutt is always comprised of more than one breed, it is usually the healthiest.
