there must be a bit of truth in the genetics passed down bit. i cant see how dogs can be much different to humans in the good or bad natured arena.

if the breed is a bit aggressive, and then u treat the dog poorly, you will get an extra 100 percent of aggression from the breed as opposed to treating the dog well and it being placid.  u dont see king charles cavs or poodles used as guard dogs etc but u do see dobermans and german sheps and pig dogs/pitbulls /staffys used for guard work or  hunting etc so i guess some breeds do have a bit more grunt than others.  but primarily even the most aggressive breads stay big puppy dogs for life if they are treated kindly from day one.  thats the hope of every dog owner i guess.lol