When medical scientists or psychologists want to understand the role of genetics and environment in human development, they often study identical twins. This is based on the idea that such twins are genetically identical, and that any differences between them in health or psychology could only be due to differences between their environments.
The scientific rationale for this belief seemed strong. Identical twins start out with the fertilization of a single egg cell in the mother’s womb. This forms a zygote with its own unique set of genes, or genome. Usually, the zygote develops into a single baby. Identical twins result when the early embryo splits and develops into two. Because they start from one zygote, identical twins are called monozygotic twins.
In 2021 a team of medical scientists from Iceland published their surprising finding that monozygotic twins aren’t always genetically identical. The researchers took blood and tissue samples from three-hundred and eighty-one pairs of monozygotic twins, and from their parents, offspring, and spouses. They determined the sequence of genetic instructions in each of these genomes, and made comparisons between them.
The researchers were looking for genetic mutations that happened in the womb, just after the splitting of the zygote that produced the twins. These mutations would make the twins genetically different, even before birth. Although they found an average of only about five of these mutations, they also found that fifteen percent of the twins differed substantially, by more than one hundred mutations.
Researchers who study the role of genes and environment in human development may no longer be able to assume that monozygotic twins are always genetically identical.