> How do we know these genes have an additive effect on IQ?
I came to post on the same topic. Some gene effects are additive but those are the simple ones and there isn't that many of them. In something complex like a human brain, there are going to be tons of interactions between genes, so that maybe genes A+B makes someone smarter and C+D also make someone smarter, but the combo A+C makes someone actually less smart. The interactions are likely to be insanely complex -- eventually solvable but it will be a while.
To use a computer metaphor, the optimization landscape is not smooth and it doesn't only have a single optimal peak, it is highly complex with lots of local hills (which are caused by the interactions.)
> The breeding of domesticated plants and animals has changed some populations by as much as 30 standard deviations. Broiler chickens, for example, have increased in size more than four times since 1957. A similar approach could be applied to human intelligence, leading to IQs greater than 1,000.
This chicken metaphor is very wrong in that that was achieved via artificial selection, not genetic engineering. Artificial selection can make directed fast movement through an optimization landscape, but it does this without actually trying to figure out the interactions at the gene level, rather it looks as the results and selectively breads for those. When you do this to human it is called eugenics, and, to put it mildly, it likely isn't going to come back into fashion: