An interesting theory on IQ and genetic load over at West Hunter. Put simply, most IQ-lowering or raising does not come from big-ticket items like Down's Syndrome, or even a few medium-ticket items we are about to discover in out new DNA-sequencing data, but from accumulations of minor genetic mutations, most of which have deleterious but nonmeasurable effects. Thus, a bell-curve distribution of "genetic noise" rather than a handful of "ooh, there's a smart one!" genes.
HT: Steve Sailer