I adopted a dog from a family where the mom was ill and also had a youngster and a newborn. The dog is half shitzu half bison. She is perfect - housebroken, affectionate, doesn't tear up things or go through the trash, can stay long periods alone, BUT when ANY new person comes around she growls and bars. I have to give everyone directions on how to approach her and ignore her until she is alright with them in my home. Once she is then she stays okay. But in my home and on the street she does that and twice she has bit people. I believe that this behavior went on before since it happened when I went to get her and the woman told me the same instructions and then said it is worse with men. She doesn't seem like an alpha personality or exhibiting alpha behavior, it seems protective. She does listen and mostly obeys commands but when a new person comes in view she starts with this behavior. Once the person has been through this she doesn't do it the next time they come around. Any suggestions or is this a big mistake on my part?