IT鈥橲 the exception that proves the rule. A massive genetics study involving over 21,000 people has revealed the best evidence yet that a single gene can raise IQ 鈥� but the effect is so small that it leaves intact the idea that variations in intelligence depend on the effects of hundreds of genes.
There is little dispute that genetics accounts for a large amount of the variation in people鈥檚 intelligence, but studies have consistently failed to find any single genes that have a substantial impact.
That has now changed, following a brain study on an unprecedented scale designed to probe the links between genetics and some brain diseases (see 鈥淏rain-drain genes鈥�). Variation in a single gene, called HMGA2, has a measurable effect on intelligence 鈥� although the gene alters IQ by just 1.29 points.
Advertisement
鈥淚t seems like the biggest single-gene impact we know of that affects IQ. But it鈥檚 not a massive effect on IQ overall,鈥� says at the University of California, Los Angeles, who led the collaboration of 207 researchers. Indeed, according to some researchers, the finding is de facto proof that intelligence relies on the action of a multitude of genes after all.
HMGA2 has previously been linked with people鈥檚 height. The IQ difference shows up in a mutation of the gene in which a single DNA 鈥渓etter鈥� is changed from C (cytosine) to T (thymine).
鈥淐 is the good one,鈥� says Thompson. As well as raising IQ by 1.29, it increases the overall volume of the brain by 0.58 per cent of average brain size. 鈥淚t鈥檚 a loss or gain of about 2 teaspoons,鈥� he says.
The team took DNA samples from 21,151 adults and also scanned each volunteer鈥檚 brain to look for size differences. It was the brain-size-altering effect of HMGA2 that led the researchers to study its impact on IQ.
They looked in more detail at a subset of 1642 volunteers from a twin study in Brisbane, Australia, who had all taken standard IQ tests. From that analysis, they discovered the 1.29 point rise associated with HMGA2鈥榮 C-variant. If people inherit that variant from both parents they have double the effect: a rise in IQ of about 2.6 (Nature Genetics, ).
鈥淚t鈥檚 important they鈥檝e found this gene, but it took a sample of 21,000 people to find it, precisely because the effect is so small,鈥� says at the Institute of Psychiatry in London, who was lead author of a groundbreaking study in 2007 which failed to find any single genes of disproportionate importance in intelligence. He says the new study confirms that the hunt for genes with a bigger impact on IQ is likely to fail. 鈥淚f it鈥檚 this hard to find an effect of just 1 per cent, what you鈥檙e really showing is that the 鈥榗up鈥� is 99 per cent empty.鈥�
鈥淚t took a sample of 21,000 people to find the gene鈥檚 effect, precisely because the effect is so small鈥�
, a professor of neuropsychology at Harvard University, agrees. 鈥淚t鈥檚 an important finding, assuming it holds up,鈥� he says. Pinker says that geneticists have had trouble finding single genes that cause large differences in traits that we know to be highly heritable, such as height 鈥� a problem known as the paradox of missing heritability.
鈥淲e鈥檝e always suspected that there are two resolutions of the paradox. One is that heritable traits are affected by large numbers of genes, each with a tiny effect. The other is that there are genes with moderate or large effects but they are highly uncommon,鈥� he says. 鈥淭he Thompson study is a first step in showing that at least the first alternative is true in the case of intelligence.鈥�
Brain-drain genes
It was brain diseases rather than IQ that encouraged Paul Thompson at the University of California, Los Angeles, and colleagues to amass data on more than 21,000 volunteers.
Thompson鈥檚 team discovered that the gene HMGA2 alters brain size by up to 0.58 per cent. Meanwhile a variant of another gene, TESC, can change the size of the hippocampus by up to 1.2 per cent. This learning and memory hub of the brain is known to be ravaged in Alzheimer鈥檚 disease. Given that the adult hippocampus shrinks by around 0.5 per cent per year, those people in which TESC results in a hippocampus 1.2 per cent smaller than average have effectively experienced more than two years of premature ageing, a factor that could accelerate disease.
A second major study using the same data set found other genes that reduce hippocampus size. A gene called harakiri controls cell death via apoptosis, so it may be that a faulty variant of it kills off some healthy cells. The other risky variant is of a gene called MSR3B (Nature Genetics, ).
鈥淗aving one copy of [either] risk variant had the same effect on hippocampal size as four to five years of ageing,鈥� says lead author of the study, of Boston University.