unusual mouse?

Question
I have three mice currently, I just got them a few days ago. I worked with mice in the pet store where I worked, so I have limited information about them.
Two of the mice I have are litter mates. The other is different entirely, I think she might be younger too. All three are females. Jingle, the one that isn't related, is very skinny and her tail isn't as soft as the other two. She's very gentle, but a bit lethargic. I'm not sure if she's sick. There isn't any discharge from her eyes or nose and her feces normal as well. She eats a lot, but I'm not sure if she'd drinking enough. If you could help me I'd really appreciate it.
Thanks.

Answer
Dear Kelsey,

Jingle might be sick but she might just be a different sort of mouse.  As long as she has plenty of healthy food, a wheel to run in, and fresh water, and as long as she moves around, poops, and doesn't hunch over or develop overt signs of illness, I wouldn't worry.  

It's possible that she has a genetic defect which makes her a little different.  This might shorten her life span.  I would assume the best, though.  And gentle is lovely.

Have fun with your three!

squeaks n giggles,

Natasha