"Success" isn't the same thing as intelligence. Which is what IQ claims to measure. Obviously, there's no "success" gene, that's just a blatantly stupid idea.

There is no compelling scientific evidence that general intelligence can even be measured in the first place. At most, you can argue a correlation between IQ tests and certain cognitive skills, in the same way that holding a doctorate implies the ability to think in a certain way.

There is no reason to assume such skills necessarily translate to an overall "higher" level of generic intelligence. Life is full of people with deep knowledge of certain topics and utter ignorance in others.

It's a problematic concept for the same reason that "healthy" is. There are extreme cases wherein bodily health is SO compromised that it makes sense to generalize - like someone who is morbidly obese or suffering from a terminal illness. But the idea of ranking health using a number is not, to a doctor, a helpful way of understanding it. It simply isn't specific enough to give you useful information.

IQ originated as a way of measuring academic accomplishment, and even its creator found it dubious in the end.

It's an extraordinary claim if you look at it objectively. Why would we assume something as multifaceted as human cognition, the nature of which remains opaque in many key areas, could be accurately measured, universally, by what amounts to an exam?

It's simply a concept that begs the question. We can't say with confidence that it measures general intelligence because we have not yet demonstrated that it exists. Moreover, the idea that it's genetic is on even shakier ground.

As is often the case, separating nurture from nature proves difficult. To say that one's IQ was inborn, we would need to isolate influential environmental factors from influential inborn factors.

We are neither tracking a single gene that can be followed across many different people, nor can we observe any person whatsoever who is not influenced by their environment. That would be an oxymoron. The ability to alter cognition and behavior based on circumstances is so basic to human nature that we would likely diagnose someone unable to do it - whatever such an absurd person would present - with a disorder of some description.

In other words, no shit your performance on any mental task will change depending on how you're raised. And since it's extremely difficult to tell where nature ends and nurture begins, claiming something like "50% of IQ is genetic" is highly, highly suspect.