Identical twins are nature's closest equivalent to a scientific control group. Derived from a single fertilized egg that splits into two embryos, they share nearly 100% of their DNA. Yet, as any twin—or parent of twins—can tell you, they are not identical in every respect. This fascinating divergence opens a window into the interplay between genetics, epigenetics, and environment.
Genetics: The Shared Blueprint
Genetics forms the foundation. Identical (monozygotic) twins have the same genetic code. This means they share inherited traits such as eye color, blood type, and many predispositions to health conditions. However, even this blueprint can undergo slight alterations. Somatic mutations can occur after the zygote splits, leading to small genetic differences.
Example: Some twins may have different susceptibility to certain conditions like cancer or heart disease due to post-zygotic mutations.
Epigenetics: The Flexible Script
Epigenetics refers to changes in gene expression that don't alter the DNA sequence. These changes determine how, when, and to what degree certain genes are activated. Mechanisms like DNA methylation and histone modification are influenced by life experiences, nutrition, stress, and even prenatal conditions.
Studies show that:
Identical twins become more epigenetically different as they age.
Lifestyle differences can accelerate these changes.
Example: One twin raised in a high-stress environment might show epigenetic markers associated with depression, while the other does not.
Environment: The Dynamic Modifier
Environmental factors start shaping twins before birth. Variations in placental blood flow, birth order, or early nutrition can result in different developmental outcomes. As they grow, unique life experiences (school, friends, trauma, hobbies) further shape their personalities and health profiles.
Shared vs Non-Shared Environment: Even if twins grow up together, non-shared environmental factors are often the most impactful in explaining their differences.
Example: One twin develops an autoimmune disease while the other remains unaffected, possibly due to differences in infection exposure or immune challenges.
The Interaction: A Three-Way Feedback Loop
Rather than working in isolation, genetics, epigenetics, and environment interact continuously:
Genetics provides potential.
Epigenetics regulates expression.
Environment shapes both epigenetics and experience.
This loop means that even genetically identical individuals can diverge significantly over time.
Case Studies and Research Findings
Neurodivergence: Twin studies in autism spectrum disorder (ASD) and ADHD show high but not complete concordance rates. This suggests a strong genetic component, modulated by epigenetic and environmental influences.
Psychiatric conditions: The concordance rate for schizophrenia is about 50% in identical twins, further reinforcing the need to look beyond genetics.
Chronic illnesses: One twin might suffer from fibromyalgia or chronic fatigue syndrome, potentially linked to epigenetic and immune system differences.
Conclusion
Identical twins provide a rare and valuable lens for exploring how our biology and lived experiences combine to shape who we are. Their stories remind us that even the most similar individuals are uniquely shaped by time, context, and chance.
Further Reading and References:
Fraga, M. F., et al. (2005). "Epigenetic differences arise during the lifetime of monozygotic twins." PNAS, 102(30), 10604-10609. https://doi.org/10.1073/pnas.0500398102
Petronis, A. (2010). "Epigenetics as a unifying principle in the aetiology of complex traits and diseases." Nature, 465(7299), 721-727. https://doi.org/10.1038/nature09230
van Dongen, J., et al. (2015). "Epigenetic variation in monozygotic twins: a genome-wide analysis of DNA methylation in buccal cells." Genes, 6(3), 347-356. https://doi.org/10.3390/genes5020347
Kendler, K. S., et al. (1993). "The prediction of major depression in women: toward an integrated etiologic model" American Journal of Psychiatry, 150(8), 1139-1148. https://doi.org/10.1176/ajp.150.8.1139
- Hallmayer, J., et al. (2011). "Genetic heritability and shared environmental factors among twin pairs with autism." Archives of General Psychiatry, 68(11), 1095-1102. https://doi.org/10.1001/archgenpsychiatry.2011.76
Office for Science and Society. “Identical Twins Are Not Identical.” Accessed June 23, 2025. https://www.mcgill.ca/oss/article/general-science/identical-twins-are-not-identical
Weber-Lehmann, Jacqueline, Elmar Schilling, Georg Gradl, Daniel C. Richter, Jens Wiehler, and Burkhard Rolf. “Finding the Needle in the Haystack: Differentiating ‘Identical’ Twins in Paternity Testing and Forensics by Ultra-Deep next Generation Sequencing.” Forensic Science International: Genetics 9 (March 1, 2014): 42–46. https://doi.org/10.1016/j.fsigen.2013.10.015.
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Firstly, science sources report that a new individual typically inherits between 50 and 200 new genetic mutations (de novo mutations) from their parents, and these mutations are not identical between individuals, even within the same family. These are new mutations that arise in the germline cells (sperm and egg) of the parents and are passed on to the offspring. They are not inherited from previous generations. Because these mutations occur randomly during the reproductive process, the specific mutations inherited by one individual will be different from those inherited by their siblings or other relatives. This contributes to the genetic diversity within a population. Research indicates that most de novo mutations in a child come from the sperm, with relatively fewer originating from the egg.
ReplyDeleteSecondly, While identical twins (monozygotic twins) share the same genome (genetic code) because they develop from a single fertilized egg that splits, they may not remain perfectly identical at the genetic level. They will start with the same genetic blueprint, but even in early development, mutations can arise independently in each twin, leading to some genetic differences. These differences can be very small (e.g., a few mutations) or, in rare cases, more significant. If there is not "mutation" when the zygote splits - they will begin identically. But you ignore the major source of difference between identical twins - not the instruction set - but how and when the instructions are implemented - epigenetics will dominate the rare blasocyst etc mutations: https://www.sciencedirect.com/science/article/pii/S1872497313002275
As discussed in https://www.mcgill.ca/oss/article/general-science/identical-twins-are-not-identical, when scientists recently read the DNA of 381 twin pairs, they reported that, on average, so-called identical twin pairs differed by 5.2 mutations. This is tiny on the scale of 3 billion letters but it is an average. Thirty-nine of these pairs actually differed by more than 100 mutations, while 38 pairs did not differ at all at the level of their DNA. Which does not much impact the usefulness of "Identical twins are nature's closest equivalent to a scientific control group"
Their (https://www.mcgill.ca/oss/article/general-science/identical-twins-are-not-identical) Take-home message:
- The scientific term for identical twins is “monozygotic twins” because these twins come from a single sperm fertilizing a single egg, which is called a zygote
- A recent study looking at the DNA of so-called identical twins revealed that, on average, each pair of identical twins differed by 5.2 mutations in their DNA
- So-called identical twins can also differ in the environment they are exposed to and in the epigenetic marks on their DNA which regulate how active their genes are, which can lead to one twin of a pair developing a disease while the other does not.
https://www.mcgill.ca/oss/article/general-science/identical-twins-are-not-identical