: I beg of anyone to explain to me why it makes sense that over 4 billion people sprang from a common set of genes, and yet were I to mate with my sister our child would have a good chance of being deformed / mentally handicapped, or both.
It doesnt make the slightest bit of common sense and although Im no scientist Id be willing to bet it is completely impossible. Id also be willing to bet there will be someone to give you some conveluted reason as to why they insist it is possible and did, in fact, happen.
None.