Genetic differences between identical twins could start very early in embryonic development, according to a study that researchers say has implications for studying the effects of nature versus nurturing.
Identical – or monozygotic – twins come from a single fertilized egg that splits into two. They are important research subjects because it is believed to have insignificant genetic differences. This means that when physical or behavioral differences arise, environmental factors are assumed to be the likely cause.
But the New Search (£), Published Thursday in Nature Genetics, suggests that the role of genetic factors in shaping these differences has been underestimated.
Co-author Carrie Stephanson said identical twins have traditionally been used to help researchers try to separate the influence of genes from the environment in analyzing diseases and other conditions.
So if you took identical twins who were raised apart from each other and one of them developed autism, the classic explanation is that it is due to the environment.
He said: “But this is a very dangerous conclusion,” adding that there is a possibility that the condition is caused by an early genetic mutation in one of the twins without the other.
A mutation means a change in the DNA sequence – a slight change that is neither good nor bad in nature but that can affect physical features or susceptibility to certain diseases or disorders.
Jan Domansky, a geneticist at Uppsala University in Sweden who was not involved in the new research, praised the research as a “clear and important contribution” to medical research. He said, “The implication is that we must be very careful when using twins as a model” to eliminate the influences of nature and nurturing.
Stefanson, president of the Icelandic genetics company deCODE, a subsidiary of the American pharmaceutical company Amgen, and his team sequenced the genomes of 387 identical twins and their parents, spouses and children in order to track genetic mutations.
They measured the mutations that occurred during fetal development and found that identical twins differed by an average of 5.2 early developmental mutations. In 15% of twins, the number of differentiated mutations was higher.
When a mutation occurs in the first few weeks of embryonic development, it is expected to be widespread in both the cells of an individual and those of his offspring.
In one of the pairs of twins studied, for example, there was a mutation present in all cells in one of the siblings’ body – meaning that it likely occurred very early in development – but not at all in the other twin.
Stefanson said that among the primary masses that would form individuals, “one twin consists of descendants of the cell in which the mutation occurred and nothing else,” while the other was not.
“These mutations are interesting because they allow you to start exploring the way twinning occurs.”
Given the genetic differences that exist, the term “identical” may be misleading to describe siblings. “I tend to call them monozygotic twins today more than they are identical,” Stefanson said.
Previous studies, including a 2008 research paper published in the American Journal of the Human Genetics, Identified some genetic differences between identical twins.
The new study goes beyond previous work by including the DNA of parents, children, and spouses of identical twins. This allowed the researchers to determine when genetic mutations occurred in two different types of cells: those in one individual and those inherited by that person’s children.
Nancy Segal, a psychologist who studies twins at California State University Fullerton and was not involved in the research, described the research as “heroic and really important”.
“This will force scientists to refine our thinking about the effects of genetics and the environment,” she said. “Twins are very similar but not perfect alike.”
With Agence France-Presse and The Associated Press
• This article was corrected on January 8, 2021. Autism is not a disease as mentioned earlier.