[QUOTE="jules21158, post: 1164, member: 9"]My son has a Jack Russell called Sid but we believe he is a cross as his head is large like a alsations and he has a bigger body than my other son's Jack Russell.\n\nSid is now becoming aggressive towards members of the family and I wondered if this was a direct result of cross breeding?[/QUOTE]\n\nI highly dout it to be honest, are you sure he is not being exposed to a stressing enviroment. Most of the time when a dog is starting to develop an aggressive behaviour it means something is going on. You need to find out what is really going on. Does he have contact with little children unsupervised?