Identical twins, or monozygotic twins, begin life as a single fertilized egg that splits early in development, giving them nearly identical genomes. While this shared DNA suggests perfect similarity, scientific understanding reveals the picture is more complex. Even with the same genetic blueprint, differences accumulate before birth and are amplified throughout life. This divergence in appearance, known as phenotypic discordance, stems from subtle variations in the DNA sequence, how that DNA is used, and the influence of the environment both inside and outside the womb.
Genetic Identity and Somatic Mutations
The foundational assumption that identical twins are 100% genetically identical holds true only at the moment the egg splits. Once the embryos develop, cell division introduces minute, random copying errors known as postzygotic somatic mutations (PZMs). These changes occur in body cells after fertilization and are not passed down to offspring.
Because these mutations occur randomly and independently in each twin, the twins diverge genetically almost immediately. Studies suggest identical twins can differ by an average of 5.2 mutations early in life. In approximately 15% of pairs, one twin has a substantial number of mutations that the other does not, which can affect various traits.
Epigenetic Differences in Gene Expression
The primary biological mechanism explaining differences in identical twins lies in epigenetics, which involves changes in gene activity without altering the underlying DNA sequence. The epigenome determines which genes are turned “on” or “off.” These instructions include DNA methylation, where chemical tags are added to the DNA, and histone modification, where proteins that DNA wraps around are altered.
Identical twins start with indistinguishable epigenetic tags, but these tags are highly responsive to environmental factors, causing profiles to “drift” over time. As twins age and experience different diets, sun exposure, physical activity, or stressors, their individual experiences create unique epigenetic patterns. This divergence means that even if both twins possess a gene for a trait, the degree to which it is expressed can differ significantly. Variations in gene expression between older identical twin pairs can be up to four times greater than those observed in young pairs.
Varied Conditions During Fetal Development
Differences in appearance and health often begin long before birth, while the twins are still in the womb. The arrangement of the placenta and amniotic sacs, known as chorionicity and amnionicity, introduces a powerful environmental variable. Roughly 60 to 70% of identical twins are monochorionic, meaning they share a single placenta.
When twins share a placenta, resources like blood and nutrients are not always distributed equally, leading to unequal resource allocation. One twin may receive a disproportionately larger share, resulting in differences in birth weight, organ size, and physical development that can persist throughout life. An extreme example is Twin-to-Twin Transfusion Syndrome (TTTS), where blood is shunted from one twin to the other, creating a significant difference in size and health noticeable at birth.
Accumulated Effects of External Environment
After birth, the non-shared external environment acts as a continuous force, accumulating small physical differences over decades. Even twins raised in the same household do not experience the world identically, leading to subtle variations in muscle development or bone density.
Differences in lifestyle choices, such as diet, exercise routines, and habits like smoking or sun exposure, directly influence the physical phenotype. For instance, a twin who spends more time outdoors will develop more sun damage and wrinkles, while a twin who adopts a different diet may experience different rates of aging and changes in body composition. Furthermore, unique injuries like scars, broken noses, or a chipped tooth create permanent, distinguishing marks that accentuate the divergence from their genetically matched sibling.