When in utero, they instruct the developing embryo in how to develop. There's gene combinations that encode for every feature the developing organism will possess. [Too pat an explanation. He wonders if he's doing her a disservice, amends it.] That's the simple explanation anyway. It's not as clean cut as I make it sound.

...Meryl and Vash? I know them. It's not easy, but it has its own kind of austere beauty and the society it creates is unique.