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Genetics of intelligence: many, many genes with tiny effects

| September 11, 2014

With the exception of the APOE gene and Alzheimer’s disease, the search for genes that control intelligence, cognitive ability and cognitive decline in aging have been largely unfruitful. That’s likely because intelligence is such a complicated human trait. Even the standardized way its measured, in terms of general intelligence, IQ, is quite controversial.

In a recent study, some very big names in the field of neuroscience and genetics published the results of an extremely large study (more than 100,000 international participants) that went looking for genes associated with cognitive performance. They found three. More accurately, they found three single nucleotide polymorphisms (SNPs) that corresponded to intelligence. And in a follow up these proved to be protective against age-related cognitive decline.

But how predictive of intelligence were these three spots on the human genome? Not very. In total the three SNPs accounted for less than 2 IQ points worth of variation in human intelligence, if a person had two copies of all the intelligence-beneficial mutations compared to someone who had none, explains Nature’s Ewan Callaway:

To put those figures in perspective, those variants have about one-twentieth the influence on intelligence as do gene variants linked to other complex traits such as height, says Daniel Benjamin, a social scientist at Cornell University in Ithaca, New York, who co-led the study.

It’s likely that there are thousands of genes with tiny effect on cognitive performance, according to the study authors. And it’s likely to take an analysis of well over 1 million humans, a study 10 times this size, to really identify those loci with certainty.

This study, however, could help point toward the biological mechanisms that underly cognitive performance. The three gene variants identified in this study, for example, are likely regulators or involved in the processes that control neural plasticity, the ability to learn and make memories, two of the authors pointed out in a editorial at the Syndey Morning Herald. That information is worthwhile for studying aging related dementia.

But for those hoping to identify intelligence genes to pass on to their progeny, blogger and geneticist Razib Khan says don’t hold your breath:

Because intelligence is at least moderately heritable, but the causal variants are so diffuse and numerous, the best bet for having a smart child is picking a spouse with a deviated phenotype. Look for smart people to marry.

Meredith Knight is a blogger for Genetic Literacy Project and a freelance science and health writer in Austin, Texas. Follow her @meremereknight.

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