In CK1 Paradox had a system whereby all stats were inherited. In the unlikely event you got a ruler with a natural, pre-traits 12 Stew and you married a woman with a similar score of 12 pre-traits, your kids scores would average 12. This led many gamers to consciously breed super-geniuses. Apparently Paradox hated that, because they totally scrapped the system in DV.
I strongly suspect the anecdotal claims that marrying only geniuses will lead to a) a whole bunch of genius children and b) many nasty mutations appearing in your line are true, because a line of half-deformed/half-brilliant rulers would really fit into the je ne se quais of this game. Moreover it would be in line with the decision I mentioned above; where they didn't only nerf breeding, they totally eliminated it from the game.