The bible is full of inconsistencies like this. If this is the word of God, stone cold facts about everything, I expect a coherent story. Incest and in breeding doesn't cut it for me. (Although that would explain DC Tom!) No I prefer to believe it is a collection of parables to help us poor idiots navigate this world and get along with each other.
And to touch on crayons Noah story, DNA would then all trace back to one Jewish descendant, Noah, it doesn't, it traces back to Africa millions of years ago, not 6000 years ago. Sorry, it is what it is. Maybe this world would be a better place if all people finally grasped this concept and put Mohammad and Jesus in their rightful places as prophets of better living and not Gods.