Okay with dogs most of it is based on training, but you can breed aggression into dogs as based on the
Cordoba Fighting Dog (fought more than fucked, thus ending breed) and the controlled experiment which favours
aggressive or friendly rats - both suggest that aggression and docility can be bred into animals. With pitbulls and other fighting/defensive breeds, often people preferred more aggressive dogs and their offspring remains in dog fighting rings around the world - while pitbulls may of been known as "Nanny Dogs" in the 19th-20th century they've been bred to be more aggro than they were originally.
But if you know anything about dog breeds and their purity you would know that a purebred is another word for a inbred dog, Pitbulls, Rotties, German Shepherds are all PALE IMITATIONS of their original, fucked up working-dog breeds because people bred in specific wanted specific traits.