THE QUESTION LIBRARY LIFE IN THE UNIVERSE
Shared DNA does not guarantee identical lives. Conditions before birth, later environments and differences in gene activity can contribute; epigenetics is one part of that picture.
WHERE THE EVIDENCE STANDS
Identical twins develop from one fertilized egg and begin with almost the same DNA. Yet their conditions before birth, later environments and gene activity can differ. Shared DNA does not guarantee identical lives.
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Not necessarily. Changes to DNA’s sequence—mutations—can arise when cells divide. A 2021 report from the researchers at deCODE describes early developmental mutations present in one twin but not the other. Differences between twins therefore cannot automatically be attributed only to their environment.
Epigenetic marks influence how cells use DNA without changing its letter sequence. Chemical tags on DNA, or on proteins around which DNA wraps, help regulate gene activity. These patterns can change during development and throughout life. Environmental exposures can also affect them.
In a 2005 twin study, Fraga and colleagues compared chemical marks in twins of different ages. Older pairs generally showed greater differences; pairs who had spent less of their lives together also differed more. This was an observational comparison, not an experiment assigning lifestyles. The association does not prove that a particular experience caused a mark, or that a mark caused a personality difference.
Marks can be copied when cells divide. Some can also pass between generations, but much of the epigenome is reset during reproduction. Inheritance between cells and inheritance of a parent’s acquired experience are different questions.
An exposure during pregnancy can directly reach a fetus and its developing reproductive cells. Effects in a child or grandchild can therefore involve direct exposure, rather than a mark transmitted beyond exposed generations.
To demonstrate such transmission, researchers must distinguish it from shared DNA, environments and culture. Horsthemke’s 2018 critical commentary explains these difficulties in human studies. A family resemblance does not establish that memories or life experiences were biologically inherited. The stronger claim needs evidence identifying a transmitted factor and showing what it does.
For the broader question of DNA similarity within our species, see how closely related all humans are.
How closely related are all humans?
Very closely.
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