Researchers have identified 26 new genes linked to intellectual disability which is characterised by significant limitations in learning.

More than one in 100 children worldwide is affected by intellectual disability. Frequently, intellectual disability also accompanies symptoms of autism spectrum disorders, and many genes have been found to be shared by the two illnesses.

The study, published online in the journal Molecular Psychiatry, could eventually lead to personalised treatments for affected individuals, and also add to our growing knowledge of brain development and functioning.

"Knowing the genes involved is a big step forward, but understanding how they function is also crucial before we can start planning treatments or even cures," said team leader John Vincent from the Centre for Addiction and Mental Health in Toronto, Canada.

The study, which was jointly led with Muhammad Ayub of Queen's University in Canada, involved 192 families from Pakistan and Iran with more than one affected family member.

Intellectual disability is frequently caused by recessive genes, meaning that an affected child gets a defective copy of the gene from each parent.

The families in the study all had a history of marriage among relatives, which occurs quite commonly in communities in South Asia, the Middle East and Africa.

Studying families with this background, and multiple affected individuals, can enable researchers to identify disease genes that would otherwise remain hidden.

The research team pinpointed mutations related to intellectual disability in half of these 192 families.

The identification of 26 new genes adds to 11 new genes that the team had previously linked to intellectual disability.

One immediate implication of the study is to prevent future cases of intellectual disability, the researchers note.

Unaffected family members and relatives could be genetically screened to see if they carry these mutations.

While 26 genes may seem a substantial number, there are likely hundreds of genes that, when defective, may lead to intellectual disability, the researchers pointed out.

"The strategy we have used speeds up the process of identifying disease genes and of enabling diagnostic labs to deliver more accurate information for clinicians and families," Vincent said.