My friend has a pit. Most gentle dog you could ever know. Even when it was provoked by someone it knows. His father used to come home drunk and kick it, never did a thing. Just tucked it's tail between it's legs and relaxed it's ears a bit until it got a "I'm sorry bud" pet from him.
One day they were all sleeping and someone broke into their house, dog went haywire and attacked the intruder. Literally launched itself off a flight of stairs onto them, biting as soon as it landed. After he had him subdued he instinctively grabbed hold of his arm and wouldn't let go, growling and snarling the entire time until the police arrived. Only problem was getting him to let go. After that he got a steak dinner every third Saturday of every month, and gained the nickname "White Fang" from me. (Read the book, you will know what I'm talking about)
I also had a Dalmatian, very over protective dog. Do you want to ban Fire Dogs as well?
She bit before, ripped my uncles hand open. He shouldn't have burst through the door. About a week after that, she was sitting next to him getting petted every time he came over. They still liked eachother. She also bit me before, and I had her since a pup when I was 3 years old. She didn't puncture the skin, didn't even hurt. Just one of those "Here's my teeth now behave" bites. I'm sure that other dogs, pitbulls, do the same thing. Although they may have alot harder "warning nips" and the people react negatively, either screaming and yelling or hitting the dog. This makes them feel threatened. How would you react when your scared? Even running away could make the dog think your playing, until you start freaking out and they feel threatened. It's all about how it was raised, peoples reactions, and provocations, not about the dogs genetics, that only plays a VERY small part.