It is hard for me to find an interpretation of "normal variation in IQ" that makes much sense of what you are saying. Also, I note that that phrase was used in an example.
Intelligence has a genetic basis. We know that some things breaking cause large effects - and that other things breaking cause small effects. There will be a bunch of genes that cause effects of intermediate size when they break - and some of those will be broken in relatively normal people - accounting for some of their variation in intelligence.
You could argue that researchers haven't yet found any such genes - but my impression is that there is a growing list of likely genes - from things like the "IQ QTL Project". My interpretation would be that neither theoretical considerations nor empirical studies offer much support to the idea that genes with moderate effects on IQ are particularly uncommon.
No doubt a fair number of such things will actually be caused by genes affecting the gut, muscles, senses, etc - but that gets back to what is meant by a gene "for" intelligence.
"but my impression is that there is a growing list of likely genes"
As I said in my first comment, the list is characterized by non-reproduced associations. I have tracked this sort of research for about 10 years now, and the pattern is a consistent one where a QTL makes a big splash, but there is no follow up. As I also stated, I have friends looking for QTLs which effect normal variation. This is a well known issue in the behavior genetics community.
"My interpretation would be that neither theoretical conssiderations nor empirical studies o...
Link.
"Razib Khan has an academic background in the biological sciences, and has worked for many years in software. He is an Unz Foundation Junior Fellow. He lives in the western United States."
Razib's writings can be found on his blog, Gene Expression.