My biting female rat

Question
I bought two 9 week old female rats for company for my current rat about two weeks a go. i bought them from a breeder, and she said that she had handled them from birth, therefore i assumed they would be friendly. One is very friendly and has never bitten once, however the other one bites if i either put my hand near the cage and even if i put my hand in the cage near her. I have had rats before but i have never experienced a rat that has bitten for no reason. i was wondering if you could give me some advice on how to stop her from biting?

Answer
 I'm guessing these rats are sisters (correct me if I am wrong).  It's uncommon but not unheard of for rats even from the same litter to exhibit extremely different personalities, even if they were raised exactly the same way.  As a breeder I've experienced this firsthand with a litter I handled from birth.  Every rat in the litter was a perfect little darling except for the lone female who slowly turned aggressive from about 4-5 weeks old and got to the point where she was so bad I had to have her put to sleep.  I had this happen again later on, but the behaviour wasn't quite as extreme and thankfully the rat eventually grew out of it.

 It's possible that the bitey rat just needs a bit more time to settle in.  Maybe she is taking the move a bit harder than her sister.  Don't give up on her, keep trying to handle her, and when she does nip at you let out a loud squeak and pull away to let her know that it hurts.  Some rats are very 'mouthy', especially young ones, just like puppies or even human babies and will nip a bit too hard by accident.  It can easily be mistaken for aggression, but it's hard to tell through an email if she is deliberately being aggressive or is play-biting.

 If she is just under stress or is play-biting, she will likely eventually grow out of it.  Squeak when she bites you, make sure your hands are clean before trying to handle her, and allow her to come to you instead of chasing after her and frightening her and she should eventually come around.

 If her behaviour is true aggression unfortunately there isn't a whole lot that can be done.  Rats who bite out of fear can often be tamed into trusting you and will quit the biting when they are no longer scared, but when they are NOT afraid of you and still bite you it is often a behaviour that cannot be altered.  For more information please see the following link: http://ratfanclub.org/trust.html

 I hope she eventually comes around!  Good luck!