No, even identical twins are not 100% genetically identical. While identical, or monozygotic (MZ), twins develop from a single fertilized egg and share nearly identical DNA, minor genetic differences arise during development. These differences can accumulate over time, leading to variations in their genetic makeup. Further, while they share nearly identical DNA sequences, epigenetic differences and environmental influences also play significant roles in shaping their traits and characteristics.
Identical Twins: Near, But Not Perfect, Genetic Matches
Many sources state that identical twins share approximately 100% of their genes (Twin Study Design - PMC, MedlinePlus Genetics, Monozygotic Twins - an overview | ScienceDirect Topics). However, this is a simplification. While they start with identical genetic material, mutations can occur during early embryonic development, leading to slight variations in their DNA. These mutations, along with epigenetic modifications (changes in gene expression without alterations to the DNA sequence itself), contribute to observable differences between identical twins, such as variations in fingerprint patterns (ELI5: If identical twins share 100% of their DNA with each other, how ...).
Fraternal Twins: Shared Genetic Material
Fraternal, or dizygotic (DZ), twins, on the other hand, develop from two separate fertilized eggs. They share, on average, only 50% of their genes, similar to any other siblings (Identical, or monozygotic (MZ), twins have 100 percent of their genes).
The Importance of Epigenetics and Environment
Even in identical twins, the environment plays a crucial role in shaping differences. While they share the same genetic blueprint, experiences and lifestyle choices lead to epigenetic modifications that affect gene expression. This is why even genetically nearly identical twins can exhibit distinct traits, susceptibilities to diseases, and overall physical appearances. (Identical twins don't share 100% of their DNA | Live Science, Identical Twins Are Not Identical | Office for Science and Society ...).
