Older fathers are the main source of faulty genes linked to intellectual disability, a recent Dutch study indicates.

The intellectual disability, associated with chromosomal abnormalities caused by missing, repeated or reshuffled DNA sequences, manifests as low IQ.

The study shows the genetic defects mainly arise anew in fathers because of the way sperm cells continue to divide and renew themselves throughout a man's life. The older the man, the greater the chances of genetic mishaps. Sperm cells carrying the faulty genes pass them onto the offspring.

In contrast, female egg cells 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. The findings are published online in the Journal of Medical Genetics.

The Dutch scientists found the group was twice as likely to have "new," or de novo, CNVs which had not been passed down through generations, as in people with autism. New CNVs were also three times more common in people with intellectual disability than in people with asthma.

Further analysis of CNV origin in 118 people for whom data were available showed that 90 got them from the father. Three quarters involved missing DNA sequences.

The authors, led by Dr. Jayne Hehir-Kwa, from the University of Nijmegen, wrote: "In conclusion, our data provide for the first time convincing evidence that CNVs in ID (intellectual disability) are largely paternal in origin."