Have you heard of Chromosome 16? No, it is not an upcoming science-fiction film but stark reality where a missing copy of a region in Chromosome 16 in the human body can result in a 25-point intelligence quotient (IQ)-drop in the carrier, claim 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.
"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 Dr Sebastien Jacquemont, a geneticist and clinical researcher at CHU Sainte-Justine health centre affiliated to University of Montreal.
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," Jacquemont noted in an article published in the renowned scientific journal JAMA Psychiatry.
To reach these conclusions, the team measured the intelligence of 700 family members who had at least one relative carrying the same genetic mutation on chromosome 16.
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Even in the participants whose IQ was considered to be normal, the researchers found a substantial 25 points IQ drop induced by gene deletions.
"Indeed, it is quite common for mutation carriers to show no mental health problems," the study noted.
Each first-degree relative (parents, offspring and siblings) has 50 percent of their genetic code in common and, therefore, 50 percent of the genetic factors that partially determine cognition," Dr Jacquemont explained.
Further studies are needed to quantify the effect of all mutations associated with autism and characterise the additive effects that lead to this psychiatric disorder.
"No single mutation can cause the whole set of clinical signs shown by these patients," the scientist added.