Why Identical Twins are Not Absolutely Identical

Identical twins are so-called because they have identical genomes, the full complement of genetic material that determines their sex and characteristics.  Despite the fact that they carry identical sets of chromosomes, identical twins still exhibit subtle differences in their physical appearance, behavior, personalities, and disease susceptibility.  Scientists are baffled.

A recently concluded eight-year study conducted by researchers from Sydney’s Garvan Institute of Medical Research and Queensland Medical Research Institute reveals that these subtle differences can be attributed to the methylation of the epigenome, a gene subset whose “voice” is either silenced or amplified by the attachment of methyl groups (hydrocarbon molecules).

Researchers Dr. Marcel Coolen and Professor Susan Clark, both epigeneticisists from Garvan, concentrated on a particular group of ‘imprinted’ genes, that are involved in controlling growth during early development, and their methylation profiles.  They found that even in identical twins, these imprinted genes have different methylation profiles, and they concluded that it is these differences that are probably giving rise to the differences between identical twins.

In the study, the researchers looked at and compared the methylation profiles of 128 identical twin pairs and 128 non-identical twin pairs to find out how closely their profiles matched.  Professor Susan Clark says that the study’s aim was to understand how genetics and environmental factors determine who we are.

“Our findings support the hypothesis that changes in the methylation reflect the interplay between the environment and genetics,” explains Professor Clark.  “We showed that methylation patterns are exquisitely inherited, and so the methylation patterns of identical twins are still very similar to each other.  This demonstrated that the DNA sequence does instruct the methylation pattern.  When that methylation pattern changes, however, it gives rise to potential changes in phenotype, or who we are.”

Study results also provide evidence that differences in methylation profiles of genetically identical people may be the cause of the differences in their susceptibility to diseases.  The findings are published online in the International journal, PLoS One.