To further their knowledge on epigenetics, scientists study identical twins, who are genetically the same. How is it that twins, who have the same exact copy of each other's genetic structure, can become so different from each other as they age? Or how is it possible that one twin will develop a specific disorder but the other won't? The answers are hidden in the studies of epigenetics. This research is focused on differentiating between genetic factors and environmental factors (26). To do this, they compare the "concordance rate" between monozygotic (MZ) and dizygotic twins (26). What made identical develop different traits? To answer this question, researchers set out to "explore the putative molecular mechanisms underlying the phyenotypic discordance of monozygotic twins" (27).
The Twin Theory
Advances in the field of Molecular Genetics allowed scientists to be able to further exam traits in twins. Identical twins develop from a single zygote, therefore, they have the same genome. This eliminates genetics as a variable telling scientists that the differences they observe between the individuals are “caused almost solely by environmental factors”. Because they are genetically identical but their environments become more unique as they age, identical twins are “excellent models for studying how environment and genes interact […] this has become increasingly important when studying complex behaviors and diseases.” In recent studies, many of them have shown that these environmentally induced differences are acquired via the epigenome. (51)
Scientists have found that 35% of twin pairs had significant differences in their DNA and that are caused environmental factors. It can alter gene expression, thus increasing their susceptibility to disease by affecting epigenetics. DNA is associated with proteins, called histones, to form chromatin. Any chemical modification to the DNA or the histones can alter the structure of the chromatin, without necessarily changing the nucleotide sequence of the DNA. In a study conducted on 80 sets of identical twins, between ages three and 74, they explored the role of epigenetics in generating phenotypical differences between identical twins. They analysed the "twins' global DNA methylation and histone H3 and H4 acetylation in samples from lymphocytes, buccal mucosal epithelial cells, skeletal muscle biopsies, and subcutaneous fat." This research produced evidence that supported the idea that "while young twins were genetically indistinguishable, older ones displayed major differences in the type and distribution of their genetic information, resulting in unique gene activation profiles.” It also observed that the biggest genetic differences were observed in twin pairs who had spent less time together during their lives or had different medical histories. “Beyond its potential importance for understanding differences in identical twins, epigenetics could explain many of the twists of fate that affect ordinary people – why one person may be struck down by cancer, for example, while another is spared, even though neither's DNA harbours a cancer-causing mutation.” (52)
Chromosome 3 pairs in each set of twins are digitally superimposed. One twin's epigenetic tags are dyed red and the other twin's tags are dyed green. When red and green overlap, that region shows up as yellow. The 50-year old twins have more epigenetic tags in different places than do 3-year-old twins. (51)