The Mother of Faces said that she had made faces since the beginning of time. I wondered if she had made all faces, since she was able to give them to any organism or object. My big question, which I started a thread about (http://avatar.wikia.com/wiki/Thread:1207592) , was if she made all human faces as well. Then, someone brought up that if the Mother of Faces did make all faces, where genetics come into play?
I did some heavy thinking about this. One of my purposes for joining the website was to see what part genetics played in determining whether or not someone is a bender. So when this question was posed, I was like, 'I gotta get in on that'.
The Mother of Faces is Koh's mother. Avatar Roku said in ATLA that Koh was one of the oldest spirits in the Spirit World. This means that the Mother of Faces is very old, perhaps as old as Vaatu and Raava. Since the spirits seem to remember everything that has ever happened in their lifetimes, we can assume that the Mother of Faces also remembers everything.
Where I'm going with this is to say that the Mother of Faces remembers all of the faces she's ever created. So she can remember each and every feature of every specific family (for example, a nose shape that is common in the family). So my theory is that genetics in faces are taken into account. Just spiritually and not scientifically.