FWIW my friend had a pit Chase for 10 years. Calmest, coolest dog I knew. One day out of the blue, it decided to attack her in her sleep, mauled her face and tore her jaw right of her face. $500K later she's back to looking normal again. She is the sweetest person I know. Nobody can explain what happened.

I look at it as 80/20. 80% of the behavior is bread by teaching. 20% is bred by genetics. Some of the most hostile behavior is within that 20% that can trigger a reaction. And if you have a purebreed, you've twisted the genetics even more against your favor.