Loxetta
Pro Adventurer
Either. Or both.
It's actually one of CC's logic gaps -- he was right there in the Mako reactor at Nibelheim where JENOVA was being housed, yet he didn't take any of her cells.
The only possible way to explain Genesis' stupidity then is that he didn't take JENOVA's cells then because he had wanted Sephiroth's cooperation as part of some recreation/fulfillment of Loveless.
Or

EDIT: Also, I love the idea of Genesis being Gast or Ifalna's son. Given that the CC Complete Guide seems to put Ifalna in Midgar prior to Gast's "escape" from Shin-Ra (says she escaped with him), it could easily be written that Genesis was her son biologically.
It would be sketchy to pull off. I have two theories

** Ifalna consented to having Gillian's genes injected into Genesis early in the pregnancy and was a willing participant in the whole Jenova project. This is unlikely since it's shown in the original FF7 that she knew full well what Jenova was. A possible circumvention would be that ShinRa didn't tell her all the details on Jenova.
** Or, ShinRa had her against her will and released her when Genesis was deemed a failure. This... opens up a lot of ugly doors, such as how Ifalna got pregnant with Genesis in the first place. So I'm not sure Sqex would touch something like this one.

Both of these kinda hit a brick wall with "ShinRa already had a real Ancient, and still wanted to go through with the Jenova project?!"
But I suppose she could have hidden that fact from them.
