Researchers at deCODE genetics in Iceland sequenced the genomes of monozygotic twin pairs in enough depth to catch mutations that arose after the fertilized egg began dividing, before and around the moment it split into two embryos.
On average, twin pairs differed by 5.2 mutations that had occurred early in development, and in about 15 percent of pairs, one twin carried a substantial number of mutations the other did not have at all, traceable to how cells were sorted between the two embryos.
The finding matters most for twin studies used across genetics and medicine, which have long assumed that any difference between identical twins must come from environment rather than DNA; some of that difference, it turns out, may be genetic. The mutations identified were mostly harmless, but the same process could in principle carry a disease-causing variant to only one twin.
Read the original study
Jonsson et al., Nature Genetics, 2021 · doi.org/10.1038/s41588-020-00755-1
Identical twins start life slightly different after all.
Source
- Jonsson, H., Magnusdottir, E., Eggertsson, H. P., Stefansson, O. A., Arnadottir, G. A., Eiriksson, O., Zink, F., Helgason, E. A., Jonsdottir, I., Gylfason, A., Jonasdottir, A., Jonasdottir, A., Beyter, D., Steingrimsdottir, T., Norddahl, G. L., Magnusson, O. T., Masson, G., Halldorsson, B. V., Thorsteinsdottir, U., . . . Stefansson, K. (2021). Differences between germline genomes of monozygotic twins. Nature Genetics, 53(1), 27–34. https://doi.org/10.1038/s41588-020-00755-1