Gotcha. I know NOTHING about rat genetics so I have no idea what that means but that sounds cool, lol! I can follow you on the Rex part of things but the silver fawn...is that a dominant or recessive trait? Mine are feeders...I have some Rex, double Rex, Dumbos and then when it comes to the colors, welllllll...that's where I get lost, lol! Regardless, glad she threw a nice litter like she did and is working out for you.