I'm not sure that's a very helpful thing to think about though. You don't have any reasonable expectation of controlling that.
I think one thing to consider is your scenario is personal choices made by the individual. If they have genius level IQ at least one of their parents has to have a significantly above average IQ and is, on the balance of probabilities, likely interested in educating their child.
But let's take the hypothetical that the home environment isn't encouraging at all. When you raise kids you don't spend every waking moment with them, there is plenty of time you leave them to their own devices and they choose what they do based on what's available to them and what interests them. Kids pester their parents pretty damn hard to get what they want. Let's assume there are pretty cold and uncaring parents. Well you will probably be in daycare, so you're not at home all the time and the chance you also go to a wretched daycare is not that high, so assume you are exploring and developing your intellect there. Then you go to school and that continue s. Maybe you are too far ahead and get bored and are diagnosed with a learning disability. As you get older you gain more freedom. Perhaps you discover the library and it makes you feel wonderful that you can learn so much and you don't get such good feelings anywhere else, so you spend more and more of your time there reading and nurturing yourself. Maybe, just maybe, you nurture yourself into a genius still.
It's not necessary.
The parents comment still stands I think. You kinda have to ask is it always the case that even in a bad home genius always manifests where it might? Or do you sometimes find that it goes entirely to waste. I'd venture to say it sometimes does go entirely to waste. On balance those kids probably go on to do well above average and it's likely you've never heard 99.999% of their names.
I'm not sure why people want to focus on outlier geniuses so much when debating the claim that there is something they as a parent can do to systematically engineer higher intelligence in their own children. I just don't think it's that useful.
I'm describing a particular scenario, and I'm not sure your studies have any data about it - the situation where a gifted child is developing in an indifferent environment (not being stimulated or motivated). No one claims we can turn anyone into a genius. But I'm pretty sure one needs very fortunate conditions to become one.