Yes , believe me I know all about JR's. Ours has never been aggresive to humans or cats but he is definatley the Alpha Male when it comes to the other four dogs, two of wich weigh 10 times as much as he does.
Our Jack Russell was aggressive with both my husband and myself. He is an alpha male and very hard headed. It took us two years to knock some sense into him. He no longer bites us but he attacks all 3 of my Rottweilers. They steer clear of him. When he bites he locks on and it is very difficult to pry him loose.