Older fathers are the chief source of faulty genes linked to intellectual disability, a study has found.

The mental problem, characterised by low IQ, is associated with chromosomal abnormalities caused by missing, repeated or reshuffled DNA sequences.

The research shows the genetic defects mainly arise anew in fathers, especially those who are older. Sperm cells carrying the faults pass them onto a father’s children.

Scientists believe the defects occur because of the way sperm cells continue to divide and renew themselves throughout a man’s life, increasing the chances of genetic mistakes creeping in.

Female egg cells, on the other hand, stop replicating during foetal development in the womb after about 30 generations.

Chromosomal abnormalities due to missing, repeated, inverted or misplaced DNA sequences are known as copy number variations.

Scientists in The Netherlands investigated the prevalence of CNVs among almost 3,500 people with intellectual disabilities.

They found the group was twice as likely to have “new” CNVs which had not been passed down through generations as people with autism. New CNVs were also three times more common in people with intellectual disability than in people with asthma.