"Genexing" is a form of Petz hexing where the "looks chromosome" is hexed to change the "genetics". You can learn more about the "looks chromosome" on Reflet's Genetics Guide.
The process I use for creating genexed litters is I take AC petz and insert the desired genes. The petz will look the same but their genes will look totally different. The magic happens when you breed them:
The offspring will expressed the edited genes in fun and unique ways. Unlike traditional hexed petz, they will "breed true" without any overwrites in.