Do twins, who often share a striking physical resemblance, also possess an identical genetic makeup? The answer is nuanced, depending entirely on how the twins were conceived and the biological events that unfolded throughout their lives. While one type of twin starts with a genetic blueprint no more alike than typical siblings, the other begins with a near-perfect genetic match that subtly diverges over time.
Genetic Similarity in Fraternal Twins
Fraternal twins, also known as dizygotic twins, originate from two completely separate biological events. This occurs when two different eggs are released during the same ovulatory cycle and are fertilized by two different sperm cells, leading to the development of two distinct zygotes simultaneously. Because they result from separate fertilization events, the genetic relationship between fraternal twins is the same as that between any non-twin siblings. They share approximately 50% of their genetic material. Consequently, fraternal twins can be of different sexes and may not look any more alike than siblings born years apart. Their separate genetic starting points mean the question of having “different DNA” is straightforwardly answered with yes.
Baseline DNA of Identical Twins
The genetic story for identical twins, or monozygotic twins, starts from a single, unified source. This process begins with one egg fertilized by one sperm, creating a single zygote. In the early stages of development, this single zygote spontaneously splits into two separate embryos, which then develop independently. Because they arise from the exact same fertilized egg, identical twins are born with a baseline genetic blueprint that is virtually 100% shared. This near-perfect genetic match means they will always be the same sex and share an extremely high degree of physical similarity. This shared initial DNA is what makes them invaluable to scientific research.
Reasons Identical Twins Acquire Different DNA
Despite the near-100% match at conception, identical twins acquire subtle genetic differences as they develop and age. These differences begin to accumulate even before birth, primarily due to a process known as somatic mutation. Somatic mutations are random changes to the DNA sequence that occur in body cells after the zygote has split.
As cells divide and multiply throughout the body, errors in DNA replication can introduce new, unshared mutations in each twin. A 2021 study analyzing hundreds of identical twin pairs found that, on average, twins differed by about 5.2 post-zygotic mutations that occurred early in development. In some cases, the difference can be substantial, potentially leading to differences in disease susceptibility or other traits.
Genetic differences also manifest through epigenetic variation, which affects how genes are expressed without altering the underlying DNA sequence. Epigenetic mechanisms, such as DNA methylation, determine which genes are “switched on” or “switched off.” While young identical twins have very similar epigenetic profiles, these patterns diverge as they age due to differing environmental exposures, such as diet, stress, or exposure to chemicals. Environmental influences in the womb can also introduce early distinctions. Ultimately, the accumulation of somatic mutations and epigenetic changes ensures that no two people, even identical twins, are truly 100% genetically identical throughout their lives.