When Yusuke Urameshi had died for the first time, he was not strong enough for this recessive strain to express itself. Ironically, through his employment as Spirit Detective, he became strong enough for this demonic trait to 'accept' him and express itself after he died in his second battle against Shinobu Sensui. As revealed by Ootake, leader of the Special Defense Force, it was forty-four generations prior to Yusuke's birth, about seven-hundred years ago, when Raizen had impregnated Yusuke's ancestral mother.