The following is the introduction of this article: Researchers often use twins as natural samples to test hypotheses regarding the contribution of genetic factors to different phenotypes, especially diseases. The classical method is comparing traits in identical, or monozygotic (MZ) twins to those of dizygotic (DZ) twins. This method has had a significant impact on our current understanding of etiologic factors in many diseases which do not follow simple Mendelian law (i.e. complex diseases), including schizophrenia, bipolar disease, and major depression.
Pomerantseva, M. (2011). Epigenetics as an Explanation for Phenotypic Variation in Monozygotic Twins. The Science Journal of the Lander College of Arts and Sciences, 4 (2). Retrieved from https://touroscholar.touro.edu/cgi/viewcontent.cgi?article=1165&context=sjlcas