Ignoring the giant walls of texts arguing inbreeding...
What is the hidden gene? Rats from breeders and petstores can carry a variety of markings,colors and coat types that won't show up unless bred to other rats with those specific genes. I've bred rats for a while and have talked to many long time breeders. Unless you have one color and repeatedly inbreed over generations to breed out anything else they could be carrying, most rat litters are pretty unpredictable with the exception of dominant colors (or sometimes color points). I just bred a Russian Blue variegated and a Blue Beige who came from seperate breeders. The Blue beige carries hairless, but the breeder of the Russian blue (to my knowledge) has no hairless rats in their collection(currently), yet about a quarter of her litter had hairless babies in it. Honestly, thats part of the fun about breeding them. With bps, there are only a couple of morphs possible, and you usually know exactly what the possibilities are. Whereas, you can spend years reading about recessive colors and marking combinations but still be unprepared when your litter colors up.