It's true that genetic mistakes can appear in newer lineages but I'm not sure whether there even was a literal Adam and Eve. It's hard for me to mesh that with other things I know.
If there was no Adam and Chava then why even be concerned with people having illicit relationships? The issue, from the Jewish perspective, is NOT genetic diseases.... The issue is that it is WRONG because our G-d told us it is wrong. And he also is the one who created mankind, through Adam and Chava...
I will not debate the science with you. But it seems to me you are denying the truth of Torah. And that is why I respond like this.
Whether the story is an allegory, or it is an actual event which occured, is immaterial. Hashem created mankind with a specific goal, and he created a Jewish people in order to bring about a change in the world to recognize Hashems hand in all of creation. You are certainly free to believe what you want.