Genetic and Environmental Influences

Ray (2018) noted that twin studies provide the opportunity to observe an event in nature that helps facilitate the understanding of important factors associated with genetic influences. The researcher explained that there are two types of twins. For example, the researcher pointed out that monozygotic twins are identical twins that have been produced from the fertilized egg, or the zygot during the first two weeks of gestation. The genes are identical as they are produced from the same egg. In contrast, Dizygotic twins result from two different eggs fertilized by two distinct sperm. These are fraternal twins as their shared genes are 50% similar to two siblings. The researcher explained how classic research design studies compare and contrast the responses of MZ twins and DZ twins on certain behavioral traits such as for example, intelligence or personality characteristics. As DZ and MZ twins it would be assumed would have been raised in similar environmental influences within the family, the differences between the two would be viewed to be the result of genetic influences. The researcher referred to a study by Gottesman (1991) in which schizophrenia was studied. In these studies, there would be a greater likelihood of an MZ
twin having schizophrenia if the other twin also did. In contrast, in DZ twins this was not so. The researcher pointed out that statistically speaking, researchers investigate the statistical degree to which twins are identical as part of a “function of genetic influences and environmental influences” (p. 119). Adoption study is another critical type of behavioral genetics research. In this situation, DZ and MZ twins have not been raised together but in separate families. In research undertaken with more than 100 pairs of twins, it was discovered that there was a 70 percent variance in IQ that could be associated to genetic factors. Studies undertaken later were able to support that specific finding. The researcher noted that if the family lived in poverty, the degree of association decreased greatly.
DiLalla and Gottesman (1995) noted in their study concerning twins with disorders that the findings showed evidence of, “personality trait differences” (p.) among twins with schizophrenia and co-twins functioning normally. The researcher noted this was a reflection of the damaging impact of schizophrenia on a normal personality as well as the
impact on co-twins having a twin sibling with schizophrenia.