But before sin had a chance to mess things up bad, people could marry a close relative and not worry about the effects to their kids. Once sin and death started adding flaws to a person's genetic code, God in His wisdom made some rules against marrying close relatives. God is SO SMART!
YAY FOR HALF ASSED SCIENCE... GET THIS:
This means that the entrance of sin into the world caused humanity's once-perfect "genes" (the stuff in you that determines things like height, eye-color, looks, intelligence, etc.) to become more and more imperfect with time.
Since close relatives have similar flaws, then the flaw would be highlighted and emphasized in any kids they might have. For instance, if two close relatives had the characteristic of a really big nose, then chances are if they married and had a kid -- he would have a super shnoz that would make Pinocchio jealous.---
I recently found the Skeptics Annotated Bible... and i must say, it's fantastic!