New study shows gene link to IQ

THE RELATIVE extent to which human intelligence is determined by our genes and by our environment is an old debate – the “nature versus nurture” debate. The consensus, based largely on studies of twins, is that about 50 per cent of the variance in intelligence in the population is attributable to genetics and 50 per cent to environmental factors.

Studies have consistently failed to identify any direct effect of a single gene on intelligence. However, the latest large-scale study, published on April 15th in Nature Genetics, has identified a single gene that has a small but measurable effect on intelligence.

It is difficult to define exactly what constitutes intelligence. One well-respected definition was given by David Wechsler (1896–1981), who developed the two most widely used assessments of intelligence scales: “Intelligence is the aggregate or global capacity of an individual to act purposefully, to think rationally and to deal effectively with the environment.” An intelligent quotient (IQ) is a score measured by a standardised test devised to assess intelligence. IQ tests are constructed so that the average score within an age group is 100 and the standard deviation (SD) is 15. Some 95 per cent of the IQ scores will lie within two SDs of the mean, ie IQs between 70 and 130. IQ scores are widely used as predictors of educational achievement or special needs. This latest study was a worldwide collaboration of 207 researchers, led by Paul Thompson of the University of California. The project analysed MRI brain scans and DNA samples from 21,151 adults and identified a single gene that has a small effect on intelligence called HMGA2.

A gene is made of DNA, a long molecule made of units called nucleotides. There are four different types, labelled A, T, G and C. The gene carries genetic information coded in the sequence spelled out by these four letters, arranged in three-letter “words” that are linked sequentially along the DNA molecule. The nucleotide letter present in any location is subject to change, leading to the possibility of gene variants.

We each inherit two copies of every gene we carry – one copy coming from our mother and one from our father. One location in the HMGA2 gene can carry either a C or a T nucleotide. We therefore each have the possibility of inheriting either two “C genes”, one “C” and one “T gene”, or two “T genes”, depending on our parents’ HMGA2 genetic characteristics. About one quarter of the population inherit two C genes, half inherits one C gene plus one T gene, and one quarter inherit two T genes.

The C variant correlates positively both with brain volume and IQ. People who receive two C genes from their parents score, on average, 1.29 points higher in IQ tests than people who receive only one C gene, who in turn score 1.29 points higher than people who receive no C gene. One C gene also increases brain volume by 0.58 per cent over people who receive no C gene. Although this study has identified one gene connected with intelligence, the effect is small and it seems likely that the overall genetic contribution to intelligence relies on large numbers of genes, each having a small effect.

The hippocampus in the brain is thought to be the seat of learning and memory. Thompson’s team study also found that variants of another gene called TESC alter the size of the hippocampus by 1.2 per cent above or below average. The adult hippocampus shrinks in volume by 0.5 per cent per annum so one copy of the wrong gene variant would contribute 2.5 years of ageing and two copies will contribute five years of ageing. This could accelerate the onset of dementia or depression, both related to hippocampus shrinkage. The good news is that exercise counters shrinkage of the hippocampus and can neutralize the effects of TESC variants.

Research on the heritability of intelligence is a sensitive area. There are reports that environment is far more important than genetics in determining exceptional ability than in determining average ability. However, the latest research, published in Behaviour Genetics (Vol. 34, No.4, 2009), strongly indicates that the influence of genes is similar across the entire range of intelligence.

William Reville is an emeritus professor of biochemistry and public awareness of science officer at UCC. understandingscience.ucc.ie