I believe it's much clearer from the text that the "genetic manipulation" started with the sons of God (fallen angels) taking wives from the daughters of men, ie, angels mated with human women. This is also the explicit explanation from the oldest version of the book of Enoch (c1st Century BC).
Now how that bloodline continued after the flood, I think the above is a good explanation. Otherwise you're left with a non-global flood, which the text does not support.
I believe it's much clearer from the text that the "genetic manipulation" started with the sons of God (fallen angels) taking wives from the daughters of men, ie, angels mated with human women. This is also the explicit explanation from the oldest version of the book of Enoch (c1st Century BC).
Now how that bloodline continued after the flood, I think the above is a good explanation. Otherwise you're left with a non-global flood, which the text does not support.