Identical Twins Accumulate Genetic Differences in the Womb

DNA replication errors during cell division cause monozygotic twins to diverge from each other even during the earliest stages of development, a new study finds.

Rather than having exactly the same DNA sequences, twins start accumulating genetic variation from the earliest stages of development, researchers at Iceland-based company deCODE genetics found, meaning that one twin harbors variants that aren’t present in the other.

Also known as monozygotic twins because they develop from a single fertilized egg, identical twins have long been central to research on the relative effects of genes and environment—aka “nature versus nurture.” Although everyone accumulates some genetic mutations during their lifetime, the differences in identical twins were assumed to be minimal, particularly when twins are young, allowing researchers to study how different environments influence the development of people with the same genotype.

The new study focuses specifically on mutations that occur as or before embryos form from the mass of cells inside the blastocyst, a structure that implants in the uterine wall. During this stage of development, this inner cell mass can split to form two separately developing embryos.

Drawing on deCODE’s genetic databases to analyze variation in multiple cells sampled from 381 monozygotic twin pairs and their immediate family members, the researchers pinpointed a number of mutations that arose during this stage of development in just one member of each pair, meaning that at birth, so-called identical twins may already differ genetically from each other.

Kári Stefánsson, CEO and founder of deCODE and a coauthor on the new study, spoke to The Scientist about how he and his team managed to identify these mutations, and about what the findings mean for research in genetics and developmental biology. 


Catherine Offord (January 7, 2021). Identical Twins Accumulate Genetic Differences in the Womb. TheScientist. Retrieved from