It appears that the hunt for genes affecting intelligence is not going well. I can't say that I'm surprised, because I have always maintained that intelligence is an emergent property of a set of co-operating genes during development in a particular environment and I don't anticipate that the geno-centric approach will take us closer to understanding it.
Intelligence, and -I believe- other complex traits are like complex dishes with many ingredients. The ingredients themselves (e.g., salt, lettuce, or chicken) are themselves unremarkable, but it is the way that they are put together and turned on and off by internal and external stimuli (the pot, the temperature, time, etc.) that makes a good dish.
Intelligence (Article in Press)
The quest for quantitative trait loci associated with intelligence
Robert Plomin et al.
Progress towards identifying quantitative trait loci (QTLs) for complex traits like intelligence and common disorders like mental retardation has been slower than expected. An important factor is that most QTL effects may be much smaller than expected—not just 1% effect sizes but perhaps effects as small as .1%. If so, this would mean that studies have been seriously underpowered to detect and to replicate QTL effects. We have used microarrays to genotype DNA pooled for groups of low versus high intelligence in order to screen very large numbers of single nucleotide polymorphisms (SNPs) on very large samples in the quest for QTLs of very small effect size: We find no effect sizes greater than .5%. Microarrays with 500,000 SNPs are now available that facilitate genomewide scans which will make it possible to identify nearly all SNP associations that account for 1% of the variance of intelligence—if there are any QTL effect sizes as large as 1%.