It was once widely accepted that identical (monozygotic) twins shared the same DNA. If identical twins were to have health differences, it was always explained by environmental factors. However, it was recently determined that identical twins will have 5.2 genetic differences on average. In Iceland, researchers examined the genomes of 381 identical twin pairs. Of the 381 pairs of twins, only 38 were exact genetic copies. Shockingly, 39 pairs of twins from the study had more than 100 genetic differences. The majority of identical twins had several genetic differences. These genetic differences are hypothesized to be from early in development. These differences may have occurred right before the egg split into two, or shortly after. Another explanation for why twins may have so many genetic differences could be determined by how many cells from the original embryo separate to form the second offspring. The less cells it takes to form a separate offspring, the more genetic variation may arise between the twins. This is shocking to most, since identical twins normally look like carbon copies. However, many of the identical twins we know may not be 100% genetically identical afterall.
H. Jonsson et al. Differences between germline genomes of monozygotic twins. Nature Genetics. Published online January 7, 2021. doi: 10.1038/s41588-020-00755-1.
Saey, T. H. (2021, January 07). Some identical twins don't have identical DNA. Retrieved from https://www.sciencenews.org/article/some-identical-twins-dont-have-identical-dna-genetics