I prefer to believe the theory of Tabula rasa anything else would make a mockery of life and the choices a person makes, and would take away from individual achievement, obviously genetics do play a part in the direction an entire species goes but unless the fate of the human race is for everyone to be the best F1 drivers in the universe I don't believe that direction will turn out to be motorsport....
Genetics plays a part, most likely, but just as likely is the wealth to fund early racing, and the fact that a famous name opens doors that wouldn't normally open, plays just as big a factor if not more.
On the Brabham front there is a new Brabham (Matty) who is suppose to be pretty quick but whether we'll see him in F1 I don't know. David never really got a shot in a good team in F1 so its hard to tell on that one.
As for the Schumacher legacy. We have two brothers who won F1 races and we currently have a son of one of those (Mick) who is apparently winging his way to F1 to form a dynasty.
Speaking of which there is a new Fittapaldi (Pietro) currently doing the rounds in lower formulas too.
Lets not forget Jolyon Palmers younger brother Will in F4, as well as Oliver Panis's son driving in Eurocup.
I'm not sure all of these are an argument for genetics but rather an argument for oppotunity and money.