Identical twins are not as similar as we think

The study found that identical twins differed by an average of 5.2 initial mutations. This finding has led scientists to rethink the view that the physical and behavioral differences between twins are due to environmental factors.

The researchers suggest that genetic differences between identical twins (identical twins) may begin very early in embryonic development.

Identical twins – also known as monozygotic twins – occur when a fertilized egg divides into two embryos and from there develop into two separate individuals. This process begins at an early stage, when the embryo is just a bunch of cells. They are important subjects of study because they are thought to have minimal genetic differences. This means that environmental factors are often cited as the cause of physical or behavioral differences.

Identical twins are not as similar as we think
This study goes further than previous studies in that they looked at the DNA of the parents, children and spouses of identical twins. (Photo: verywellfamily).

But a new study, just published in the journal Nature genetics, suggests that scientists have underestimated the role genetic factors play in shaping these differences.

Kari Stefansson, co-author of the study, said that identical twins are often the traditional subject of research by scientists as they try to separate genetic and environmental influences on the process of differentiation. illness and other conditions. “So if there’s an identical twin growing up together, and one of them has autism, the classical explanation is environmental.”

“But it’s an extremely dangerous conclusion,” he said, adding that the condition could be caused by an early genetic mutation in one of the twins, and not in the other.

Mutation, which can be understood as a change in the DNA sequence – a small change that is not inherently good or bad but can affect physical characteristics or susceptibility to certain diseases or disorders certain disorder.

Jan Dumanski, a geneticist at Uppsala University in Sweden who was not involved in the study, hailed the study as “a clear and important contribution” to medicine. “ This means we have to be very careful when modeling twins ” to separate the effects of birth from nurture.

Stefansson, head of Iceland’s deCODE genetics division, a subsidiary of the American pharmaceutical company Amgene, and his colleagues sequenced the genomes of 378 identical twins and their parents, husband and wife. , their children to monitor gene mutations.

They looked at mutations that occurred during embryonic development and found that identical twins differed by an average of 5.2 early developmental mutations. In 15% of twins, the number of mutations is higher.

When a mutation occurs during the first few weeks of embryonic development, it is likely to spread in both the individual’s cells and the cells of their offspring. For example, in one of the twins studied, a mutation was present in every cell in a person’s body – meaning it may have occurred very early in development – but completely absent from the other.

Stefansson thinks that these early mutations will form individuals, “one of the twins is formed from the ‘descendants’ of the cell where the mutation occurred” , while the other does not. “These mutations are exciting because they allow you to start exploring ways to create twins.”

In addition, because of this difference, scientists also question whether the term “identical” should be used to describe another set of twins. ” Now I tend to call them ‘monozygotic twins’ rather than ‘identical’,” Stefansson said.

Previous studies, including one published in 2008 in the American Journal of Human Genetics, have identified some genetic differences between identical twins. The new study goes beyond this because they also looked at the DNA of the parents, children and spouses of identical twins. That allows the scientists to determine when gene mutations occur in two different types of cells: those that occur only in an individual, and those inherited by that person’s descendants.

Nancy Segal, a psychologist specializing in twins at California State University Fullerton who was not involved in the study, called the work a major achievement and “really important.”

“This will force scientists to adjust their thinking about genetic and environmental influences,” she said. “Twins are very similar but not exactly alike.”