Given the genetic differences found, the very term identical may be misleading to describe the siblings.
Identical twins form from the same egg and get the same genetic material from their parents.
Simply put, identical twins may not be so identical after all.
The new research suggests that identical twins had, on average, 5.2 genetic mutational differences between them.
When a mutation occurs in the first weeks of the embryo's development, it tends to be found in all the cells of an individual (somatic cells such as those of muscles, organs, etc., but also cells reproductive or gametes), and to be transmitted to its offspring.
The implications of this are significant, according to Stefansson, as the research led the team to conclude that "the role of genetic factors" in shaping the differences observed between monozygotic twins "has been underestimated".
The scientists found two groups of twin pairs, one where twins share mutations and another where the developmental mutations are only present in one of them at high frequency. "Our work shows it is unsafe to jump to that conclusion".
We have found a twin pair where one of the twins has mutations in all cells of his body, and they are not found in any cell in the body of the other twin.
H. Jonsson et al., "Differences between germline genomes of monozygotic twins", Nat Gen, doi:10.1038/s41588-020-00755-1, 2021. They also found mutations that occurred before the developing embryo split into two, setting the stage for twins.
"As cells replicate, and replicating machinery makes a copy of the genome - it's not flawless", Stefansson said.
© Ott et al./Microbiome, 2020 Transmission electron microscopy image of a D. radiodurans cell splitting.
"These mutations are interesting because they allow you to begin to explore the way in which twinning happens".
After an egg is fertilized and becomes a zygote, somatic mutations can begin popping up within days, once the zygote's cells begin dividing to form a mass of cells called a morula.
In 15% of them, the number of these different early mutations is even "substantial", underline the authors.
Fraternal or dizygotic twins are formed by two eggs that are fertilised by two sperm and produce two genetically unique children. But before you can make that interpretation, you'd better make sure that one of them does not have a de novo mutation in an important gene that the other one does not.
"This is an extraordinary, exciting and insightful effort to pinpoint early cellular mechanisms that explain genetic differences between MZ (monozygotic) twins", said Nancy Segal, an author and professor of psychology who studies twins at California State University, Fullerton, and was not involved in the research. "Then we found twins where a mutation was found in all cells in the body in one twin, but in only 20% of the cells of the other", said Stefansson, the founder and CEO of DeCode Genetics, which is a subsidiary of the USA pharmaceutical company Amgen.