Reading ability in the general population may be influenced by a gene linked to dyslexia, a new study shows.

The KIAA0319 gene is the prime suspect thought to be behind the learning disorder affecting up to one in 10 people.

Dyslexia disrupts the development of literacy and language skills, such as reading and spelling, but has no impact on general intelligence.

At least six genes have been identified that might be involved in dyslexia.

The most likely candidate is KIAA0319, carried by 15% of the population. Variants of the gene have been associated with dyslexia in large samples of British families and twins in the US.

Now new findings from a study of more than 6,000 British children aged seven to nine have indicated that faulty KIAA0319 may reduce general reading ability in people not suffering from dyslexia.

Dr Silvia Paracchini, from the Wellcome Trust Centre for Human Genetics at Oxford University, who led the research, said: "On average, people carrying this common genetic variant tended to perform poorly on tests of reading ability. However, it’s important to note that this is only true for reading ability and not for IQ, so it doesn’t appear to be connected to cognitive impairment."

The children were part of a wider investigation known as the Avon Longitudinal Study of Parents and Children (Alspac), an ongoing research project from the University of Bristol.

A total of 14,000 mothers were enrolled into the study in 1991/92. Scientists have been closely monitoring the health and development of them and their children ever since. It was already known that a particular DNA pattern, or haplotype, in one variant of the KIAA0319 gene is associated with reduced activity of the gene during development in the womb.

The mutation affects development of the cerebral cortex, the part of the brain responsible for thought processes.

In animal foetus studies, switching off KIAA0319 hinders the ability of nerve cells created in the inner layer of the cerebral cortex area to migrate out to their destination.

Reading tests conducted as part of the new study revealed a statistically significant lowering of performance in children with the haplotype.

"This is clearly only part of the jigsaw puzzle that explains why some people have poorer reading ability than others or develop dyslexia," said Dr Paracchini.

"There are likely to be many other contributing factors, but our research provides some valuable clues. We need to carry out studies into the exact role that this gene plays in brain development and how this affects people’s reading ability."

Professor Margaret Snowling, vice president of the British Dyslexia Association, said: "The finding of a ‘gene’ associated with reading ability in the general population as well as in dyslexia is in line with our observation that there are degrees of dyslexia from mild to severe.

"It also implies that other genes or environmental experiences must be involved in determining reading ability.

"This ties in with what we have known for many years — some individuals are able to compensate and go on to successful careers even though they carry this gene variation.

"Another exciting finding is that this ‘gene’ seems associated with slow brain growth in certain regions. From a practical perspective this research underlines the need for the early identification of dyslexia and suggests that intervention at a time when the brain is still developing could lead to positive outcomes for literacy and other skills."

In the tests, children were asked to read out loud 20 words of increasing difficulty. Their performance was rated according to how long they took and how many mistakes they made.

About 13% of the children did not reach a pre-set target level of reading ability.

Spelling was also assessed based on the ability of children to spell 15 words of increasing difficulty.

In addition, parents filled in questionnaires asking about any reading difficulties their children might have. About 6% of the children had needed special help with their reading at school.

Scores obtained from these assessments were compared with a population average. Children with the KIAA0319 gene variant consistently obtained lower than average ratings.

Dr Paracchini stressed that although the effect seen was statistically significant, the real impact on children’s ability to read and spell was slight.

"It’s a very small effect. What is interesting is that it’s very consistent. People with this gene variant score below the average, even when they are not dyslexic.

"The suggestion is that if you have this gene variant plus other susceptibility factors, which may also be genetic, the result might be dyslexia. But this factor alone, maybe combined with other effects, can still have an influence.

"What is important is that now we have a firmer basis for thinking that this gene is the one contributing to dyslexia."