Failing to find genes for intelligence has, in itself, been very instructive for Plomin. Twin studies continue to persuade him that the genes exist. “There is ultimately DNA variation responsible for it,” he says. But each of the variations detected so far only makes a tiny contribution to differences in intelligence. “I think nobody thought that the biggest effects would account for less than 1 percent,” Plomin points out.
That means that there must be hundreds—perhaps thousands—of genes that together produce the full range of gene-based variation in intelligence.
Variation in intelligence thus seems to be an accumulation of small effects from very many genes. For a long time, I had trouble reconciling this view with the concept of g—this single mental property, still unknown, that accounts for most variation in human intelligence. I now realize that this contradiction is more apparent than real. Whatever this single property might be, it doesn’t necessarily correspond to a single gene or even a single gene complex. It could be affected by an indefinite number of genes.
Zimmer, C. (2008). The search for intelligence. Scientific American, October, pp. 68-75.