[tt] 'Intelligence genes' reveal their complexity

[Hughes, James J.]

http://www.newscientist.com/channel/being-human/mg19626324.100-intellige
nce-genes-reveal-their-complexity.html

'Intelligence genes' reveal their complexity

    * 29 December 2007
    * From New Scientist Print Edition. Subscribe and get 4 free issues.
    * Andy Coghlan

Something as subtle and complex as intelligence was never going to be
pinned on just a handful of genes, as a huge trawl across the human
genome seems to confirm. Although it did turn up hundreds of genes that
make a contribution, their individual effects are so small that for the
most part they are barely detectable. This does not mean, however, that
intelligence is not inherited.

The research, led by Robert Plomin of the Institute of Psychiatry in
London, identified six genes that were strongly associated with high or
low intelligence, but even the most powerful of these accounted for just
0.4 per cent of the variation in intelligence between individuals. The
six together accounted for about 1 per cent of the variation in
intelligence. Dozens of previous studies on twins and adopted children
have established that about half of the variation in intelligence is
down to environment, but almost all of the genetic component has yet to
be uncovered.

"If we've creamed off the biggest ones, and they only account for 1 per
cent of the variance, then there's a long way to go," Plomin says. "The
most striking result is that there are no large effects."

In their experiment, Plomin and his colleagues obtained intelligence
scores for 7000 7-year-olds, based on verbal and non-verbal reasoning
tests. They also took DNA samples from the children. To find regions of
DNA that correlated with high or low intelligence, they screened DNA
from the children who scored highest or lowest in the tests against a
gene chip coated with 500,000 single nucleotide polymorphisms (SNPs) -
regions of DNA that differ between individuals. This enabled the team to
identify hundreds of SNPs that differed between the high and low
scorers.

Next, they whittled the significant SNPs down to just 37 that correlated
most strongly with intelligence, then looked at those SNPs in the DNA
from children across the whole intelligence range. This process revealed
the six highly significant gene regions (Genes, Brain and Behaviour,
DOI: 10.1111/j.1601-183X.2007.00368.x).

Most researchers contacted by New Scientist were not the least bit
surprised that the genetic aspects of a complicated trait like
intelligence are down to the cumulative effects of a wide combination of
genes. "Intelligence is a function of the way the brain is put together,
and at least half of our genome contributes in some way or another to
brain function, which means that in order to build a human brain, you
need thousands of genes to work together," says Gary Marcus, a
psychologist at New York University.

Neuropsychologist Stephen Pinker at Harvard University warns that the
study should not be interpreted as evidence that intelligence is not
heritable. "There are many ways in which genes could affect intelligence
other than single genes with large effects," he says.

Pinker is confident researchers will soon build on Plomin's findings.
"Whole-genome scans are in their infancy, and a variety of molecular and
statistical improvements will soon tell us how the heritability of
intelligence - which is not in reasonable doubt except by politically
motivated critics - is related to variations in the genome."

Indeed, Katherine Burdick of the Zucker Hillside Hospital in Glen Oaks,
New York, claims to have found genes with a greater impact on
intelligence than those identified by Plomin, even though she screened
the same 500,000 SNPs. "Our top three SNPs have [cumulative] effects in
the range of 9 to 10 per cent [of variance]," she says. Burdick is due
to present her research in Boca Raton, Florida, at the annual meeting of
the American College of Neuropsychopharmacology.

Plomin is sceptical: "Until people publish, you must take things with a
large pinch of salt."

Genetics - Keep up with the pace in our continually updated special
report.
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Weblinks

    * Genes, Brain and Behaviour
    * http://www.ingentaconnect.com/content/mksg/gbb
    * Robert Plomin homepage
    * http://www.robertplomin.com/
    * Stephen Pinker, Harvard University
    * http://pinker.wjh.harvard.edu/research/index.html
    * American College of Neuropsychopharmacology annual meeting
    * http://www.acnp.org/Default.aspx?Page=2007Registration