Identical Twins, Identical Fate?
Epigenetics is the study of the chemical reactions that activate and deactivate parts of the genome at strategic times/locations, and the factors that influence them. According to the article within the case study, during an identical twin’s lifetime, epigenetic changes occur to their DNA that can affect gene expression. Therefore, one twin can express a certain trait while the other cannot. The twins still have identical DNA, but they might express different traits because of different epigenetic changes. These epigenetic changes are influenced greatly by the twins’ behaviors and environment. Temperature, for example, is a big factor. An experiment using two alike flies—both hatched with white eyes—proved that raising the temperature caused them to lay offspring with red eyes. This offspring can then pass these changes onto its own offspring. These traits can be passed on to every generation, or they can skip generations. Traits skip generations when the mother has the specific trait, but the father does not.
Epigenetic modifications are used to regulate gene expression.  The two major modifications are: chemical modifications to the cytosine residues of DNA (DNA methylation) and histone proteins associated with DNA (histone modifications). In the DNA methylation modification process, a cytosine base becomes methylated through the action of an enzyme called a DNA methyltransferase. In the histone modification process, the core histone is subject to several types of modifications, including: acetylation, methylation, phosphorylation, ubiquitination, sumoylation, etc.. Histone modifications are critical for regulating chromatin structure and function, which can in turn affect many DNA-related processes. The patterns of epigenetic modifications can show gene activity and expression, as well as chromatin state.