I know that two of my male mice have kinked tails from fighting. With one of the female mice i bought from a pet shop, I suspect that her kink was genetic since all her siblings also had kinked or shortish tails.
I'd be interested to know about the genetics behind genetically kinked tails. I have heard that kinked tails could be caused by the tailless gene. I've also heard that kinked tails were caused by the flexed-tail gene. Whether these are one and the same gene or completely different ones, I don't know.