Scientists have for the first time pinpointed a large number of genes that are reliably associated with dyslexia.
Around one-third of the 42 genetic variants identified have been previously linked to general cognitive ability and educational attainment.
Researchers said the findings, published in Nature Genetics journal, help to understand the biology behind why some children struggle to read or spell.
Dyslexia is known to run in families – partly because of genetic factors – but little was known about the specific genes that relate to the risk of it developing until now.
The study, led by the University of Edinburgh, is the largest genetic study of dyslexia to date.
Previous studies linking dyslexia to specific genes have been done on small numbers of families and the evidence was unclear, the research team says.
This latest study involved more than 50,000 adults who have been diagnosed with dyslexia and more than one million adults who have not.
Researchers tested the association between millions of genetic variants with dyslexia status and found 42 significant variants.
Some of these are associated with other neurodevelopment conditions, such as language delay, and with thinking skills and academic achievement.
Researchers say they were able to predict how well children and adults from four other research studies can read and spell using the genetic information from the study, but not with the accuracy needed for diagnostic use.
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.
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.
“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,” Dr Luciano said.
“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.”