It's been a while since I took genetics, but aren't all tortoiseshells considered chimeric?
Back when I actually WAS mouse goddess, I made chimeric mice. We'd inject genetic material (stem cells) into a blastocyst. The blastocyst was for a white mouse, and the stuff we were injecting was coded for a brown mouse, so we could see how much of the injected material "took". We'd get some really interesting mice out of it....from all white with the tiniest bit of brown, to all brown with the tiniest bit of white, but most were something in between, Dalmatian spotted mice, or mice that were half and half.