Is IQ more genetic or more behavioral?

I've read reports that identical twins with the same DNA can have IQs varying by as much as 24 points, so this seems to indicate that behavior is the bigger factor.

But others, usually racists, claim that IQ is in your DNA, but I haven't seen any real evidence for this.

Then others say that it's 50/50, but that seems like a made up speculative number from no where.

So what's the biggest factor influencing IQ?