Uh no disputing there are genetics involved, but there is a ton of stimulus involved as well. It is both. The brain is just like the rest of our body, if it is not used or "worked out" it goes to waste. Let's me use an anecdotal example to illustrate my point.
Someone could be the child of best female body builder and the best male body builder in the world. If that person sits on the couch all day, eating only candy bars and potato chips, I don't think he's going to have the body of a world leading body builder. However, the child of genetic stock not really known for muscle development and body building could be way more ripped than the couch potato with tons of genetic "potential" going to waste.
Sure their limits would be different if both people in my exampled worked their bodies out to max potential, but only maybe. This brings me to my second point of human genetics.
I hate to point this out to you, but humans are very adaptable while growing. Human genes are within 5 percent or less of each other in totality. Meaning you pick anyone person on the planet and they are going to share 95% or more of the same genes that you have. The difference is what is turned on and what is not. Genes are also not always "on" at birth or during womb development and can be activated later in life or the growth cycle. Meaning the the act of stimulus upon a developing body of a child can activate a gene sequence and their body then develops and adapts to the environmental stimulus.
Just throwing out that black people are typically stupider than white people because of genetics only is a very skewed and narrow stance to take. And it is mostly wrong. Again, all humans have basically the same genes. I posted a story the other day of two Nigerian decent Brits that gave birth to
a non albino white child This can only happen because they have the exact same genetics in their system as white people. Neither of them has any "white" or European ancestry in their genealogy either.
Now when you combine active genetic statics of typically turned on genes due to parentage with a typical environment stimulus, you are going to get stereo typical trends. But don't go spouting off that it is all genes when it is not.