Lack of a gene copy can trigger a drop in IQ levels

Toronto: Lack of a vital gene copy of Chromosome 16 may cause a 25-point drop in your IQ, a new study has claimed.

Several disorders are associated with Chromosome 16 abnormalities in the human body like a missing copy of region in this chromosome can lead to a 25-point intelligence quotient (IQ)-drop in humans, according to researchers.

Also, an addition of a copy in the same genomic region on chromosome 16 - known to predispose to autistic spectrum disorders - results in an approximate 16-point drop in IQ.

"We have just discovered, for example, that a missing copy of a region in chromosome 16 results in a 25-point intelligence quotient drop in carriers," said Sebastien Jacquemont, a clinical researcher at CHU Sainte-Justine, the mother-child hospital affiliated to University of Montreal.

"Strangely enough, even if carriers show much differentiated sets of symptoms -- and sometimes no symptoms at all -- the specific effect of these two mutations seems to remain the same," said Jacquemont.

According to him, to understand mental disorders, we must quantify the specific effect of each contributing gene mutation. No autism is alike. This is also true of most mental disorders.

"We now understand that each gene mutation has a specific effect, which adds to other effects to draw a unique picture of the disease in each patient," he wrote in an article published in the renowned scientific journal JAMA Psychiatry.

To reach these conclusions, the researchers measured the intelligence of 700 family members who had at least one relative carrying the same genetic mutation on chromosome 16.

Even in participants whose IQ was considered to be normal, the researchers found a substantial 25 points IQ drop induced by 16p11.2 gene deletions. "Indeed, it is quite common for mutation carriers to show no mental health problems," says the study.

"Intellectual faculties are the sum of many factors, the majority of which are genetic and inherited from parents. Each first-degree relative -- parents and offspring, siblings -- has 50 per cent of their genetic code in common and therefore 50 per cent of the genetic factors that partially determine cognition," said Jacquemont.

Studying families thus enabled the researchers to measure the factors that combine with the mutations which effects they wanted to quantify.

"For example, depending on the additional factors involved, a 25-point IQ drop can determine whether or not a person has crossed the threshold of 'intellectual disability."

Further studies are needed to quantify the effect of all mutations associated with autism and characterize the additive effects that lead to this psychiatric disorder. "No single mutation can cause the whole set of clinical signs shown by these patients," concludes the scientist.