The research I have seen seems to be mixed. However, it seems to depend on whether both parents have similar cognitive abilities. Each child tends to be more like one parent or the other in terms of cognitive abilities, not a mixture or average. So, if you have two gifted parents then most children will probably be gifted. If you have one gifted and one not it is a crap shoot, you could end up with all or none gifted. From the actual research I have seen environment does not necessarily make a huge difference (no more than a standard deviation) unless, there is a particularly negative (most likely abusive) environment involved.
