Identical twins have many things in common. Some of them include their gender, physical features, and even the same DNA. However, a new study suggests that in identical twins with autism spectrum disorder (ASD), the manifestation of symptoms may vary to a large extent. This is in spite of having the same genetic makeup.

According to the study, understanding the factors that lead to this variability may provide new insights into the treatment of ASD and its symptoms.

"Here we provide new evidence that although ASD itself is highly heritable, variation-in-severity of symptomatology above the diagnostic threshold is substantially influenced, in contrast, by non-shared environmental factors which may identify novel targets of early ASD amelioration," say the researchers in the study.

What is autism spectrum disorder (ASD)?

It is a neurological and developmental disorder that influences an individual's interaction with others, behaviour, and learning abilities. While it can be diagnosed at any age, symptoms typically express themselves within the first two years of an individual's life.

It affects communication and impairs social interactions with others. Studies have shown that if one of the identical twins is diagnosed with ASD, the likeliness of the other twin having the same condition is very high.

Identical Twins
Representational Picture Wikimedia Commons

Symptoms varied between twins

For the study, the researchers studied data from three previous researches which comprised of a combined total of 366 identical twin pairs who did and did not have ASD. Based on parental ratings on a standardised questionnaire, or assessment by doctors, the severity of symptoms and traits associated with autism were measured. In some cases, the diagnosis was arrived upon using both methods.

Using the findings, the researchers arrived at the consensus that there was a 96 percent chance that if one of the twins was found to have ASD, the other twin has the condition as well. What sets the diagnosis of one twin from the other is the significant variance in the symptoms among them, the study said.

Factors contributing to the variance in symptoms

The researchers dismiss genetic causes largely. They estimate that genetic factors contribute only to nine percent of the cause of varying symptoms among identical twins. In contrast to twins with ASD, the scores for traits were very similar among identical twins without ASD.

Nearly all environmental factors were also ruled out by the authors as the DNA sharing twins were raised in the same environment. They concluded that further studies are paramount in understanding the causes of the variance of ASD symptoms among identical twins.