I always tell my son: make sure you find a tall wife for having kids! Now, that all said, while it helps to have good genes, having good genes alone doesn't work. Look at how many kids of major leaguers never made it to the majors. And, there is always the first. There have been many major leaguers whose parents were not athletes. Personally, I think "before...say...1990ish" that genes alone might get you there. But, today, there's so much training and conditioning now that you need to get the work in too...to keep up with those doing it...and can't just rely on genes.
I think genetics extend beyond just height/weight/fast twitch/slow twitch etc. Quickness, eye-hand coordination, balance, ability to control heart rate, ability to focus and function when the rest of the body is in a high stress situation all have some genetic influence.
But tall, strong and fast is a really good place to start.