LITMIR.BIZ

      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, and that an organism receives one of these from each parent. Further, one of these can be dominant or recessive, which determines which characteristics are expressed. Mendel also realized that the inheritance of the gene of one trait is not affected by the inheritance of the gene for another trait. In the previous example illustrating the inheritance of color and height, those factors influencing color do not affect height, and vice versa. That is, the probability for each occurs separately. This fact is known as Mendel’s second law or the law of independent assortment.

      Since Mendel’s time, we have learned a great deal concerning the process of inheritance. What he referred to as elements or units of information, we now call genes (see Figure 2.25). We also know that genes can have alternative forms, which we call alleles. Independent researchers, Walter Sutton and Theodore Boveri, in 1903 put forth a theory we now accept as fact, that genes are carried on chromosomes. We now know that each of the approximately 20,000 human genes occurs at a specific site, called a locus, on one of our 23 different pairs of chromosomes. As genetics progressed in the twentieth century, it became clear that it was necessary to go beyond the two laws suggested by Mendel to a more complex understanding of how traits are passed from generation to generation. For example, if two genes are located close to one another on the same chromosome, then the result is different from that predicted by Mendel’s second law.