I'm astonished about how many people here are using physical traits that are observable (even with the naked eye) as analogies to brain functions. Yes, we know a lot about muscle fibers, nutrition, physics and all that stuff, we can make reasonable conclusions about physical limits, but we don't know much about our brains or minds for that matter, not even state-of-art neurology have come up with solid conclusions, yet people here are talking (and being absolutely sure, which is the worst part) about intelligence inherited by genes, brain limits...

I just can't.

You're arguing for magic. If brain function does not follow from our physical makeup, then where exactly do you think it comes from? Sure learning is like exercising a muscle and can shape your brain, but environment affects muscle growth too via exercise and nutrition. Would you deny that your genetics place an upper limit on your achievable strength?

Sorry to say, there is already plenty of evidence of direct genetic impact on cognitive ability that you can see with your own eyes, even ignoring the twin studies that all but prove that cognitive functions are largely heritable. For instance, down syndrome, Tay-Sachs disease and Leukodystrophies.

The scientific case is in: given comparable nutrition and a standard education, your intelligence as compared to your peers is determined by your genetic makeup.

This says nothing about your work ethic or your determination of course, and thus what you can achieve if you put in enough effort. Intelligence is merely a metric stating that given the same amount of work expended on cognitive problems, those with higher intelligence will do better on average.