It may be that IQ tests are not the only way to measure human intelligence. Scientists at The Queensland Institute of Medical Science (QIMR) have identified two regions of the human genome that appear to explain variation in IQ. Their work was published this week in the American Journal of Human Genetics and has drawn world-wide interest.
The research has shown that between 40% and 80% of variation in human intelligence can be attributed to genetic factors. Until now though, attempts to identify these genes have been unsuccessful and work on identification of genes responsible for genetic variation in human intelligence has been restricted to the lower end of the spectrum, concentrating on severe forms of mental retardation with neurological causes.
Human intelligence is a quantitative trait that is controlled by multiple genes. The QIMR group, led by Professor Nick Martin, has identified specific locations on chromosomes 2 and 6 as being highly influential in determining IQ. To do this, they applied multipoint linkage analysis to data from 634 sibling pairs (including non-identical twins) from Australia and the Netherlands who were genetically scanned for the study.
Although earlier twin studies had revealed the existence of genes that dictated human intelligence, usual genetic association methods had not been effective in identifying them. Where association analysis may overlook closely spaced genes that act together to affect a trait, linkage analysis is more sensitive to such combined effects.
Traditional IQ tests are designed to assess abilities across different areas – memory, vocabulary, semantics, symbolic reasoning – collectively grouped into higher orders such as verbal and performance intelligence. The region on chromosome 2 shows significant links to performance IQ, also overlapping a region associated with autism. The region on Chromosome 6 showed strong links with both full-scale and verbal IQ with a marginal overlap to an area implicated in reading disability and dyslexia.
During the study, where the non-identical twins and other sibling pairs had significant differences in IQ, they also had significant variation in these regions on chromosomes 2 and 6.
Identifying the genes responsible for variation in the range of normal intelligence could provide clues for the treatment of disorders like dyslexia, autism, attention deficit hyperactivity, and even schizophrenia.
Professor Nick Martin said “This is a good start but the next, more difficult stage is to find the specific genes in these chromosomal regions which are responsible for variation in intelligence. Knowledge of them may suggest new therapies for intellectual disability”.