While it’s commonly assumed identical twins have identical DNA and fraternal twins have distinct genetic codes, the reality is more complex. This perspective overlooks a more nuanced answer, particularly for those considered “identical,” where subtle variations can emerge over time.
How Twins Form and Their Initial DNA
Twins originate through two primary biological processes, leading to either monozygotic or dizygotic twins. Monozygotic, or identical, twins result from a single fertilized egg that splits into two separate embryos early in development. Since they arise from the same initial genetic material—one egg fertilized by one sperm—identical twins begin with nearly indistinguishable DNA sequences. They are always of the same sex because they share the same sex chromosomes.
In contrast, dizygotic, or fraternal, twins form when two separate eggs are fertilized by two different sperm during the same pregnancy. Consequently, fraternal twins share approximately 50% of their DNA, similar to any other non-twin siblings. They can be of the same or different sexes.
How Identical Twins Develop DNA Differences
Despite their shared genetic blueprint at conception, identical twins can acquire DNA differences over their lifespan through two main mechanisms: somatic mutations and epigenetic modifications. Somatic mutations are spontaneous changes in the DNA sequence that occur in body cells after fertilization and are not inherited from parents. Research indicates that identical twin pairs can differ by an average of 5.2 to 14 post-zygotic mutations, with some showing over 100. These mutations can begin very early in embryonic development, even before the zygote fully separates.
Beyond changes to the DNA sequence itself, identical twins also develop differences through epigenetic modifications. Epigenetics refers to changes in gene activity that do not involve alterations to the underlying DNA sequence. Instead, these modifications influence how genes are turned “on” or “off,” thereby affecting gene expression. Environmental factors, including diet, lifestyle, chemical exposures, and stress, play a significant role in shaping these epigenetic patterns. While young identical twins may have very similar epigenetic profiles, these differences become more pronounced as they age and are exposed to varying environmental influences.
Why DNA Differences in Twins Matter
The study of DNA differences in twins provides valuable insights into the complex interplay between genetic predispositions and environmental influences on human traits and health. Twin studies offer a unique natural experiment to disentangle the contributions of genetics and environment to various characteristics and diseases. This research helps quantify the extent to which genes or environment influence conditions.
This understanding has practical implications in both medical and forensic fields. In medicine, recognizing that identical twins are not truly genetically identical helps explain why one twin might develop a disease while the other remains healthy. This knowledge guides medical personnel in advising non-affected twins about potential risks and necessary testing. In forensics, traditional DNA profiling cannot distinguish between identical twins because their standard genetic markers are the same. However, advanced techniques like whole-genome sequencing can now detect accumulated somatic mutations unique to each twin, providing a means to differentiate them in criminal investigations.