Gender

here's my guess...mind you, it's just a guess, but...

being that men have some levels of estrogen and women have some levels of testosterone, not all the cells would turn into a gender to match their "host" body...

perhaps the reason Ozzy was turning into a girl was to balance out the estrogen levels in Christine (because if Ozzy stayed a boy, she'd have to much testosterone)

those're my thoughts :)

"Argue for your limitations, and sure enough, they're yours."~Richard Bach

Oh the plot holes.

I don't think anyone knows. I guess the animators used their "artistic licence".

Think of it this way: Hector has an X and Y chromosone in his twenty third pair, christine has two X's.
This may explain the reason the cells are all chicks, but I have no idea why Ozzy switched