new mouse gender

Question
okie hi i am 15 and i have had a female mouse for about maybe almost a yearrrishh...i JUST bought a new mouse and i am NOT sure if it is a boy or a girl. My mom said if its a boy and they have babies...haha NOT GOOD. And uhm...its deffinatley younger than the female one (her name is RONDA) and she is brown. And the new one i have now is also brown...now what im worried about is why they chase eachother and Ronda seems to be QUITE defensive against the little new one. And i don't know if they just have to like get used to each other or what but its worrying me i don't wanna go to school and come back and the little one is dead. But i don't want babies either. Even though there is an age difference they could still make babies couldn't they
PLEASE GET BACK TO ME AS SOOOOOOOOOOON AS POSSIBLE
please and thanks.

Answer
Dear Brandi,

If you don't want babies, separate the mice immediately.  If the new mouse is a male, they may have already mated.  

When mice are introduced to each other, there is often a lot of chasing and squeaking.  This is completely harmless unless

1.  Someone is bleeding, or
2.  Someone is not getting access to food, water, or the nest.

There are several ways to tell the gender of a mouse.  If the boy is old enough, he will have a puffy looking protruding rear, which is actually his testicles. But younger boys don't have that yet.  Another difference is girls have nipples and boys don't, so if the new mouse has nipples you are ok.  If you don't see them, though, it's  no guarantee, because mouse nipples can be hard to see.  Lastly, the distance between the genitals and the anal opening is more on a male mouse.  Raise them up by their tails and see if it looks the same.  

Age makes no difference to a mouse.

Good luck!

squeaks n giggles,

Natasha