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Why do identical twins have highly similar genomes?

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human genetics Hereditary information is contained in genes, which are carried on chromosomes.

Identical twins, also known as monozygotic (MZ) twins, have highly similar genomes because they originate from a single zygote—a single egg fertilized by a single sperm.12

During the early stages of embryonic development, this single zygote undergoes cleavage and spontaneously splits into two separate cell masses. Because both individuals develop from the same fertilized egg, they inherit the exact same hereditary material, sharing identical genes at every chromosome locus.132

While they are genetically identical at the start, it is important to note that they may not be perfectly "identical" throughout their lives. Minor differences can arise due to processes such as mutation or the influence of environmental factors during development, which can modify physical characteristics. Nevertheless, their shared genetic origin makes them significantly more similar to each other than fraternal twins, who develop from two separate eggs and sperm and share only about half of their genes, similar to ordinary siblings.234