Source: Reprinted from Vinod Menon, Large-Scale Brain Networks and Psychopathology: A Unifying Triple-Network Model, Trends in Cognitive Sciences, Vol. 15, pp. 483–506, Copyright © 2011, with permission from Elsevier.
connectivity: the concept that different areas of the brain work together in specific conditions
Concept Check
How does the brain operate as a “small world framework,” and why is this significant?
How is the brain’s default or intrinsic network different from the central executive and salience networks?
Researchers are concerned with modularity and connectivity in terms of neural networks. What are modularity and connectivity, and how are they important in thinking about psychopathology?
Genetics and Psychopathology
In this section, we consider the genetic level of analysis. This discussion includes a historical understanding of the study of genes as well as their structure. You will then learn about the role of DNA, how genes influence behavior, epigenetics, mitochondria, and endophenotypes.
Genes form the blueprint that determines what an organism is to become. Specific genes have been associated with a variety of disorders as will be described throughout this book. However, the original hope of finding a few genes that were involved in particular mental disorders has not panned out. What has become apparent is that there is a complex interaction of genetic and environmental factors involved in mental illness. Just having a gene does not mean that it is active—it turns on or off under a complex set of circumstances.
As the factors involved have become more complicated, there has been a search for particular processes related to psychopathology. For example, there exists a gene (SERT) that is involved in the removal of the neurotransmitter serotonin from the synapse. A variant of the SERT gene has been associated with depression, alcoholism, eating disorders, ADHD, and autism (Serretti, Calati, Mandelli, & De Ronchi, 2006). Likewise, a variant of the gene (DßH), which is associated with the synthesis of norepinephrine from dopamine, is associated with schizophrenia, cocaine-induced paranoia, depression, ADHD, and alcoholism (Cubells & Zabetian, 2004). It is suggested that the lower level of the proteins produced by the DßH gene is associated with a vulnerability to psychotic symptoms.
As researchers discover genes related to specific forms of mental illness, there may be a need to reorganize the manner in which we view mental illness. One study analyzed the genes from 33,332 individuals with a mental disorder in comparison with 27,888 without a disorder (Cross-Disorder Group of the Psychiatric Genomics Consortium, 2013). This research suggests that similar genetic risk factors involved in calcium channel signaling exist for what we have considered to be separate disorders. These five disorders are autism spectrum disorder, schizophrenia, bipolar disorder, major depressive disorder, and ADHD. This study implies that a particular genetic makeup may put some individuals at higher risk for developing a variety of disorders. There is also research that suggests that having certain mental disorders such as schizophrenia may actually protect these individuals from getting certain types of cancer (Tabarés-Seisdedos & Rubenstein, 2013).
There is a complex interaction of genetic and environmental factors involved in mental illness.
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Overall, current genetic research suggests a complicated relationship between genetic conditions and environmental factors. For example, the MAOA gene, which is located on the X chromosome, makes the neurotransmitters serotonin, norepinephrine, and dopamine inactive and is associated with aggression in mice and humans. Caspi and his colleagues (2002) performed a longitudinal study and found that mistreatment as a child influenced some boys differently from others later in adulthood. Those boys who were mistreated in childhood and had a particular form of the MAOA gene were more likely to be violent and engage in a variety of antisocial behaviors as adults, as well as have problems with law enforcement officials. Those without this particular form of the gene did not display antisocial behaviors, even if they had been mistreated as children. Thus, environmental influences in terms of maltreatment modulate the expression of specific genetic structures but not the expression of others.
As researchers studied how genes turn on and off and what factors influence this, the story became even more complicated—the processes that determine which genes turn on and off could themselves be passed on to the next generation. Of course, which factors turn the genes on and off are largely influenced by the environment of the organism. Thus, although the genes themselves could not be influenced by the environment, it was possible for the environment to influence future generations through its changes to those processes that turn genes on and off. This is referred to as epigenetics.
epigenetics: study of the mostly environmental factors that turn genes on and off and are passed on to the next generation
The Study of Genetics
The study of genetics begins with the work of Gregor Mendel (1823–1884). Being curious as to how plants obtain atypical characteristics, Mendel performed a series of experiments with the garden pea plant. Peas are a self-fertilizing plant, which means that the male and female aspects needed for reproduction develop in different parts of the same flower. Therefore, successive generations of peas are similar to their parents in terms of particular traits such as their height or the color of their flowers.
Mendel found that when combining peas that have white flowers with those with purple flowers, the next generation had all purple flowers. Allowing this generation to self-fertilize brought forth plants that had purple flowers but also some that had white flowers. Mendel explained these findings by suggesting that a plant inherits information from each parent, the male and female aspects. Mendel was hypothesizing that information must be conveyed. He further suggested that one unit of information could be dominant in comparison to the other, which we now call a recessive trait. In this case, the unit of information that coded for purple would be dominant.
Mendel did not know about genes but hypothesized the existence of a specific structure he called elements. From his experiments, he determined the basic principle that there are two elements of heredity for each trait (e.g., color in the previous example). Mendel also assumed that one of these elements can dominate the other and if the dominant element is present, then the trait will also be present. In addition, Mendel suggested that these elements can be nondominant, or recessive. For the trait to appear, both of these nondominant elements must be present. These ideas are referred to as Mendel’s first law or the law of segregation.
Mendel’s first law or the law of segregation: for the dominant trait to appear, only one dominant element is needed; for the recessive trait to appear, both nondominant elements must be present
Mendel’s second law or the law of independent assortment: the inheritance of the gene of one trait is not affected by the inheritance of the gene for another trait
chromosomes: thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique
Put in today’s language, Mendel suggested that variants of a specific gene exist, which account for variations in inherited characteristics,
