The Key Gene is a genetic make up made by Dr. Hideo, So that he and others could make there ways into the world of Planet Fumei/The Shinto Realm through the Shinto Gates. It allowed those that passed through's body to contain itself through the dimesonal rift. Without it, mortals can die by combustion if they enter without it.
The Keygene lived in most Kasaihanians and has been directly passed through the Tasanagi, and Ryoji families. Along with the Tetsu's as well. All those after the second generation within these familes have this gene. Others will have to find it at other means.