Identical twins originate from a single fertilized egg and share virtually all their genetic material, leading to the expectation of perfect sameness. However, physical differences, such as the presence or unique placement of a birthmark on one twin but not the other, are often observed. Understanding this biological paradox requires looking beyond shared DNA to examine the unpredictable nature of fetal development, which dictates the unique physical manifestation of each individual.
The Shared DNA of Identical Twins
Identical twins, or monozygotic twins, form when a single fertilized egg splits into two embryos early in development. This process gives them the same genetic blueprint and inherited genes, leading to the assumption that they are genetic carbon copies. However, the term “genetically identical” is nuanced; while their inherited DNA sequence is nearly a perfect match, minor differences accumulate early in development. On average, pairs of identical twins differ by about 5.2 mutations that occur after the zygote splits, meaning they are not 100% identical at the cellular level at birth.
The Developmental Origin of Birthmarks
Birthmarks are localized anomalies resulting from errors in cell migration or concentration during fetal development, rather than traits determined by inherited genetic code. These skin lesions fall into two categories: pigmented birthmarks, like moles, and vascular birthmarks, such as hemangiomas or port-wine stains. Pigmented marks form when melanocytes, the cells producing skin pigment, cluster abnormally. Vascular marks are caused by errors in the development or concentration of blood vessels in a specific area. The formation of both types is a highly localized event dependent on the precise timing of cell division and migration, making the exact location and size largely a product of chance.
Non-Genetic Factors That Create Differences
Somatic Mosaicism
The most direct explanation for different birthmarks is somatic mosaicism. This occurs when a mutation or developmental error happens in a cell after the fertilized egg has split into two embryos. Subsequent cell division passes the mutation to daughter cells in only one twin’s developing body, creating a unique patch of cells—the birthmark—that the other twin lacks. The timing of this initial error is crucial, as an early mutation in one twin’s cell lines results in a visibly different feature.
Intrauterine Environment and Epigenetics
The intrauterine environment also exerts unique, localized pressures on each twin. Even when sharing a placenta, differences in vascularization mean one twin may receive a slightly different distribution of blood flow and nutrients, influencing localized cell development. Another element is epigenetic divergence, which involves chemical modifications that determine how genes are read without changing the underlying DNA sequence. These markers diverge in the womb due to the distinct positioning and micro-environments of each fetus, affecting the developmental pathways of cells like melanocytes or vascular cells.