my pet rat attacked

Question
I have had my pet rat for about 6 months and was purchased from a pet shop as a sale for another customer so the age is unknown. She(they said it was a girl) is a large black and white (blade rat as the pet shop said)  and very loving and cuddly. We recently added another rat to the cage (a 5 week old) but the sex is unknown. They were fine for 1st couple of days but now the big rat keeps chasing the little one around and even bit it, and my son, when he went to get his little one out for a cuddle.We are still very new to owning pet rats and will take any advice we can get. Why would she do that to the little one???

Answer
Some rats don't like living with roommates, and would prefer to be lone rats. Her behavior suggests as much, and it might be wise to separate the baby from your current girl and get another baby (of the same sex) to accompany your newest baby. Be sure to have them of the same sex, otherwise you'll end up with an entire litter of rats, which can be very time consuming and costly as well as placing the burden on you to home them appropriately.