I also breed for temperament in my roosters. No nasty ones are allowed to stay. I have a friend that will take them for her rooster bachelor pen and then turn them into soup when they get big enough. I only allow the nice ones to stay, and because of that, only the nice ones get to breed. Easy as that for me. However, I do want to add that I think at least some of it is how they're raised. My two original roosters, a buff orp I still have, and a mostly RIR mix who passed away last winter, I raised myself from chicks. Both were the sweetest things, and even climbed into my lap to be pet. Now, the RIR mix would prefer you didn't pick him up, but otherwise he loved humans. Was never aggressive and never once showed aggressive behavior. His son, however, I let go as a day old to my friend who is not as hands on raising her birds as I am. He was crossed with sweet little EE hen who is just a darling. There was no aggressiveness ever shown from either parent. That rooster started biting and flogging my friend the second hormones kicked in. I think had he been raised a little different, he wouldn't have been like that. So while genetics play a major role, I think the way a rooster is raised plays a role too.