Identical twins, also known as monozygotic twins, originate from a single fertilized egg that splits into two separate embryos. This shared beginning often leads to a common assumption that they possess exactly the same DNA. However, whether their DNA is truly identical is more complex than a simple yes or no, involving subtle variations and dynamic biological processes.
The Shared Genetic Blueprint
Identical twins form when a single egg is fertilized by one sperm, creating a single zygote that divides into two distinct embryos. This process means that at splitting, both developing twins share virtually identical genetic material. Originating from the same blueprint, their DNA sequences are considered identical at conception. This shared genetic starting point explains why monozygotic twins are almost always the same sex and exhibit very similar physical appearances.
Subtle Genetic Variations
Despite their shared origin, the DNA sequences of identical twins can diverge subtly after conception. Spontaneous somatic mutations, changes in DNA occurring in body cells after fertilization, can arise during cell division, leading to slight differences in DNA between tissues or even between the twins. Additionally, copy number variations (CNVs), differences in the number of copies of particular genes, can emerge early in development. These genetic variations are rare but can accumulate over time, meaning identical twins are not always 100% genetically identical.
The Influence of Epigenetics
Beyond direct changes to the DNA sequence, identical twins can also differ in how their genes are expressed, a phenomenon known as epigenetics. Epigenetics involves modifications to DNA or associated proteins, which control whether genes are turned “on” or “off” without altering the underlying DNA code. While twins start with highly similar epigenetic patterns, these can diverge significantly throughout their lives. Environmental factors, including diet, lifestyle, exposure to toxins, and psychological stress, can influence these epigenetic marks. Different life experiences can lead to varied gene activity between twins, even if their DNA sequence remains the same.
How Differences Emerge
Minor DNA sequence differences and varied epigenetic profiles contribute to distinctions between identical twins. These subtle biological divergences can manifest in various ways, such as differences in disease susceptibility. Slight variations in physical appearance, like fingerprints or moles, cognitive abilities, or personality traits, can also emerge. While identical twins begin with a remarkably similar genetic blueprint, their individual life journeys and environmental interactions carve out unique biological paths for each, making them distinct individuals.