![]() In the 1940s, the idea that SCZ was due to mother–child relationships was widely accepted with Fromm-Reichman’s term, “schizophrenogenic mother”, emphasizing the role that the environment played in the onset of SCZ (Fromm-Reichman 1948). Eugen Bleuler agreed with Kraepelin’s theory but considered hallucinations and delusions to be secondary with conditions resulting from some disruption in the cognitive processing regarded as psychopathologies (Maatz and Hoff 2014). However, Kraepelin argued for the involvement of a biological component (Kendler and Engstrom 2018). ![]() At that time, the idea of urbanization and mental stress as factors that could cause psychosis in young people was widely accepted. This has had a significant impact on subsequent diagnostic classifications. In 1899, Emil Kraepelin classified psychosis into ‘dementia praecox’ and ‘manisch-depressiven Irreseins’ (Bipolar Disorder in DSM-5). Studies on heritability Heritability of SCZ The current paper focuses on epidemiological and molecular genetic research studies in MZs with the purpose of unravelling useful insights on factors that can lead to SCZ and ASD. In recent years, studies on the differences between genomic and epigenomic characteristics of MZs have led to the development of new approaches for elucidating the etiology of psychiatric disorders (Liang et al. The similarity of the genetic information in MZs provides a useful means for determining the disorder-causing role of environmental factors. As a result, researchers are beginning to think that genetic factors play a major role in the onset of these conditions. In psychiatry, there is a much higher concordance rate for schizophrenia (SCZ) and autism spectrum disorder (ASD) in MZs versus DZs (Hilker et al. ![]() Thus, genetic factors are thought to play a major role as compared to environmental factors (Rutter 2006). ![]() Since MZs are considered to share 100% of their genetic information, their concordance rate is higher compared to DZs who share about 50% of the genetic information. In epidemiological twin studies, comparisons of the concordance rate between monozygotic twins (MZs) and dizygotic twins (DZs) are very important. This line of inquiry led to the “nature versus nurture” debate that is still argued to this day (Torrey et al. At the end of the nineteenth century, Francis Golton was the first to consider twins as a useful scientific model, questioning whether human traits originated from genetic or environmental causes. In Greco-Roman mythology, the twin-pair of one human and one immortal (Castor and Pollux Gemini) served as an important example of “discordant” twins. Twins have long been a source of mystery. As a result, twin studies of psychiatric disorders are greatly contributing to the elucidation of genetic and environmental factors in the etiology of psychiatric conditions. Therefore, the potential possible effects of intestinal flora inside the body are currently being investigated as a cause of discordance in MZs. Although it was originally thought that epigenetic changes occurred as a result of environmental factors, and thus were not transmittable, it is now known that such changes might possibly be transmitted between generations. Since the epigenetic status differs in each tissue type, in addition to the DNA from the peripheral blood, studies using DNA from nerve cells induced from postmortem brains or induced pluripotent stem cells are being carried out. Although recent advances in next-generation sequencing have increased the accuracy of this line of research, there has been greater emphasis placed on epigenetic changes versus DNA sequence changes as the probable cause of discordant psychiatric disorders in MZs. Recently, biological studies based on molecular methods are now being increasingly applied to examine the differences between MZs discordance for psychiatric disorders to unravel their possible causes. The basis for these studies is that MZs share 100% of their genetic information. Twin studies of psychiatric disorders such as schizophrenia and autism spectrum disorder have employed epidemiological approaches that determine heritability by comparing the concordance rate between monozygotic twins (MZs) and dizygotic twins.
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