If you want to learn more about training and controlling your Jack Russell, my ebook is bound to save you hours of training trial and error and relieve a lot of stress from the process: The Jack Russell Lover's Ultimate Guide to Training comes complete with a full chapter on mix breeds.
My minature Jack Russell bitch is is 2 and has not been spayed has suddenly had a character change!
She has gone more clingy than normal and if you are sat on the sofa and she wants you she will bark and whine till you get up and see what she wants. She is also very hungry all the time. She is a happy go lucky little girl and that has not changed. She is not nasty in any way.
Any ideas would be helpful!