I have a mouse that was "off" as a baby. She didn't grow any fur until she was 5 weeks old, and now that she is 9 weeks old, she is still the size of a 5 week old (maybe a little bigger, but not much). I'm not expecting her to get much, if any, bigger. She will obviously never be bred, which is unfortunate, as she is my only LH self black. She is normal proportionately, and in all other ways, just tiny. She is such a little sweetie too.
I can't say for sure that it is any kind of a gene, as she hasn't been bred, and won't be, so I don't know if she just wasn't taken care of properly my momma mouse, or if there is something different about her. She is my special girl, that's for sure. So far, she has had no health problems, but i do check her frequently to make sure she is warm enough, as I found her about a month ago, cold and not wanting to move alot. I held her in my hand for a bit, and warmed her up, then took her siblings out of the cage, and gave her a little alone time with momma mouse, but she hasn't had that problem since that time.