Q&A: What causes differences between identical twins?

Identical twins share exactly the same genome, and are usually raised under the same conditions during the early parts of their lives. Thus, it is not surprising that identical twins share strikingly similar physical features. However, twins often find themselves dealing with rather different health situations as adults. An ongoing study of twins at King’s College in London has found that the many surprising differences in the lives of identical twins are largely due to the effects of a biological mechanism known as epigenetics.

Epigenetics essentially functions through methylation, the process in which a chemical add-on called a methyl group binds to our DNA. Since methylation inhibits a gene from producing a certain protein, genes that would otherwise regulate illnesses such as obesity, c

Image courtesy of Martin Schoeller
Image courtesy of Martin Schoeller

ancer, and depression may be limited or no longer expressed due to the activity of epigenetics.

Differences in behavioral tendencies of twins, such as smoking, stress, and diet, can cause changes in the methylation patterns of the epigenome. Studies have suggested that pain tolerance, for example, is not very heritable, but is instead related to variation in one’s methylation states. Ultimately, it is differences in life patterns, coupled with other environmental factors, that cause epigenetic changes and lead to striking health inconsistencies between identical twins. From these discoveries, scientists are now able to better judge which health conditions are purely heritable, and which are primarily influenced by behaviors and epigenetics.