female mouse mounting; aggression issues

Question
I just bought two fancy mice from Petsmart who ensured me that they are a female only store and only supply female mice. I brought both home and at first everything was fine and even let me hold them right away. Now the brown one who is the calmer of the two always hides in its shelters because the other one which is tan and white controls all of the cage including the wheel, food dish and water and is quite bossy plus she is extremely hyperactive and jumpy. Recently she started what I think is having sex with the brown one and making it squeak and holding onto it from behind and I try to separate them  but she just keeps trying. Is it because she is in heat? Or did Petsmart really just lie to me and sell me a male? If so, what should I do? And what do I do if the brown one gets pregnant?  

Answer
Dear Rachel,

A dominant female mouse does mount another female, so don't worry about that.

I am worried about the brown mouse though. She can't continue to live that way or she will be unhappy and depressed. If this is still true in a week, you must do something about it.

Other than separating them there is one thing you can try. Separate them into two cages for a week but keep the cages next to each other so they can communicate (you can't hear it but they do squeak to each other). After the week is up, don't clean the brown mouse's cage. Put the white mouse in the brown mouse's cage. See what happens. This is putting the white mouse at a disadvantage because she is on the other mouses turf.

If there is immediate aggression by the white mouse then take them out and put a drop of REAL vanilla extract (it must be the real stuff) on their chins and rear ends, and try again. This makes them both smell the same.

If that doesn't work, the two mice must be separated. If you can't get any more mice to keep each one company, put the cages side by side (wire cages are better than aquariums for this). In that case each mouse should be held for a minimum of an hour a day. You can probably hold them both together. They may even hang out together in a play area for short periods of time.