"The sins of the fathers (and mothers) are not visited on the sons (and daughters)."

That's a Protestant view.  The Catholic view is based on this passage from Exodus 20:5

 "You shall not bow down to them or worship them; for I, the LORD your God, am a jealous God, punishing the children for the sin of the parents to the third and fourth generation of those who hate me . . . " 

The Bible is divinely inspired text that generally explains WHY and not HOW.  Biblical history points to only 2 instances where the human genetic pool was corrupted and altered forever, given our current understanding of human genetics. 

First, by original sin (Genesis 3), which effectively shortened the lifespan of human beings, from immortal (in paradise) to 1,000+ years old (the Fall) to the current 120+ years old.  And second, by the Nephalim (Genesis 6) -- the lead theory of bible scholars:  fallen angels mating with flawed human beings -- which probably explains all the genetic anomalies that we see.