Imagine a pair of identical twins coming into the world for the first time. Some may think they are exact replicas; however, studying Identical twins have cemented the idea that human behavior is determined by nurture. This means that the twins have already produced several differences from the time the egg split. The formal term for this occurrence is epigenetics, the study of changes in organisms caused by modification of gene expression rather than alteration of the genetic code itself, which has been flagged by the wholistic and minute studies of identical twins. For example, when epigenetics is put into perspective, it is understood that “an average twin pair carries 359 genetic differences that occurred early in development”(Live Science). Therefore, it is impossible to truly determine the factors of nature versus nurture using a true study of identical twins, even if there is a control group, since the practicality of a mother giving birth to two entirely similar beings is virtually impossible. On another note, the use of monozygotic twins allows individuals to study the role of epigenetics in disease by controlling for many potential confounders, such as genetic factors, age, gender, maternal effects, cohort effects, and environmental influences, which can be highly accurate if the study is replicated.
Interestingly, since identical twins have the same genetic build, they are much more likely to be exposed to the same diseases. In fact, if identical twins are more similar to each other with respect to an illness than fraternal twins are, then vulnerability to the disease is rooted at least in part in heredity. It is necessary to note that there is an “80% chance that twins will experience the same disease in their lifetimes”(‘Identical Strangers’ Explore Nature Vs. Nurture). In fact, the majority of the study of diseases that twins endure come from the study of epigenetics, which takes into account different lifestyle and genetic encounters to determine why identical twins would differ with respect to health more so than fraternal twins. Therefore, there is not a definitive answer about if one twin, or both, could develop the same disease.
In accordance with this finding, there is also a second discovery that in addition to identical genes, there is a second factor that plays into their likeness- “an additional level of molecular similarity that influences their biological characteristics”(Live Science). This means that twins are susceptible to the same cancerous diseases. In fact, it was found that epigenetically supersimilar genes are at a high risk of developing several types of cancer that include lung, prostate and colorectal. In fact, monozygotic twins are relatively often discordant for most common complex diseases, such as type 1 diabetes, type 2 diabetes, autism, schizophrenia, and different types of cancer. This observation supports the finding that for many complex traits, genotype alone may not fully determine phenotypic variation, and the exchange between genes and environment needs to be considered. Epigenetics has been proposed to be one of the main mediators of this interaction.
Additionally, It is necessary to note that there are further factors that play into the accumulation of disease. In fact, it was found that twin pairs who were older, spent less time together, or had more dissimilar health or medical histories showed greater differences in both types of epigenetic marks. In 2009, Zachary Kaminsky, a psychiatric disease specialist at Johns Hopkins confirmed the presence of DNA methylation differences between identical twins “across three different tissues (white blood cells (WBCs), buccal epithelial cells, and gut biopsies in an age- and sex-matched twin sample”(Live Science). Interestingly, he also observed greater similarities between DNA methylation profiles within monozygotic twins than within dizygotic twins.
