SCIENTISTS have for the first time pinpointed a large number of genes that are reliably associated with dyslexia.

Dyslexia is known to run in families - partly because of genetic factors - but until now, little was known about the specific genes that relate to the risk of it developing.

It is a common learning difficulty, estimated to affect around 2.3 per cent of Scotland's school-age population. It mainly causes problems with reading, writing and spelling for reasons that are not yet fully understood.

The latest study, led by scientists at Edinburgh University, involved more than 50,000 adults who have been diagnosed with dyslexia and more than one million adults who have not.

It is the largest genetic study of dyslexia to date.

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Around a third of the 42 genetic variants identified have been previously linked to general cognitive ability and educational attainment.

The researchers said their findings, published in the journal Nature Genetics, advance our understanding of the biology behind why some children struggle to read or spell.

Previous studies linking dyslexia to specific genes have been done on small numbers of families and the evidence was unclear.

The Edinburgh research team tested the association between millions of genetic variants with dyslexia status and found 42 significant variants.

Some of these are associated with other neuro-development conditions, such as language delay, and with thinking skills and academic achievement.

Many, however, are novel and could represent genes that more specifically associate with processes essential for learning to read.

Many of the genes associated with dyslexia are also associated with attention deficit hyperactivity disorder.

A much smaller overlap of the genes associated with dyslexia was found for psychiatric, lifestyle and health conditions.

Several of the associated genetic variants were also found to be significant in a Chinese-speaking sample, suggesting that there are general cognitive processes in learning to read and write that are not dependent on the language used.

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The researchers were able to use the genetic information gathered as part of the study to predict the reading and spelling abilities of children and adults taking part in four other research studies.

However, they stressed that their predictions did not have the accuracy required for diagnostic use.

Lead researcher Michelle Luciano, of the University of Edinburgh’s School of Philosophy, Psychology and Language Sciences, said the study sheds light on many unanswered questions around dyslexia.

She said: “Our findings show that common genetic differences have very similar effects in boys and girls, and that there is a genetic link between dyslexia and ambidexterity.

“Previous work suggested some brain structures may be altered in people with dyslexia, but we did not find evidence that genes explain this.

“Our results also suggest that dyslexia is very closely genetically related to performance on reading and spelling tests reinforcing the importance of standardised testing in identifying dyslexia.”

Other key researchers in the study were from the Max Planck Institute for Psycholinguistics in the Netherlands, QIMR Berghofer Medical Research Institute in Australia, and US company 23andMe, Inc.

The difficulties associated with dyslexia were first recognised in the 1870s, but a consensus definition was not reached until the 1960s.

Dyslexia is typically given as a diagnosis if reading and spelling abilities are poor and much lower than a person's other academic skills or cognitive abilities.

Twin studies comparing incidence, where both twins grew up in the same environment, have shown that the likelihood of both twins having dyslexia is higher for identical than non-identical twins. These kinds of studies confirmed that dyslexia is strongly heritable.