BrainBrainbehavior and Biology (Genetics & Inherited Conditions)
As the first organ system to begin development and the last to be completed, the vertebrate nervous system—brain, spinal cord, and nerves—with the brain at the control, remains something of an enigma to biologists and other scientists. The vertebrate brain comprises, among other structures, neurons, which are special cells that generate and transmit bioelectrical impulses via a number of different neurotransmitters. The brain consists of three major neural structures: the brain stem, the cerebellum, and the cerebrum. A reptilian brain consists of only the brain stem, while the mammalian brain has all three, including a well-developed cerebrum (the two large hemispheres on top). The brain stem controls basic body functions such as breathing and heart rate, while the cerebrum is the ultimate control center. Consisting of billions of neurons (commonly called brain cells), the cerebrum controls such higher-level functions as memory, speech, hearing, vision, and analytical skills.
Scientists have long sought to understand the complex relationship between the brain, behavior, and genetics. Decades of research have led to a general consensus that fundamental to human behavior, cognition, and emotions is the functioning of the cerebral cortex (that is, for higher-brain functions) and the limbic system, which includes the amygdala, septum, cingulate, hippocampus, anterior thalamic nuclei, fornix, and mammillary...
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Genes and Behavior (Genetics & Inherited Conditions)
Traditionally, the field of behavioral genetics has emphasized evaluating how much population variance is determined by environmental or hereditary factors. From the perspective of human development and behavior, the issue also becomes one of how this process is expressed within cultural constraints—by what means genetic and social surroundings reciprocate to yield obvious outcomes.
Genes make proteins, and proteins cause biochemical responses in cells. The behavior of an animal takes place under the combined influences of its genes, expressed through the actions of proteins, and its environment. A good example is the phenomenon of mating seasons in many animals. As day length gradually increases toward spring and summer, a critical length is reached that signals the release of hormones that result in increased sexual activity, with the ultimate goal of seasonal mating. The production and activity of hormones involve genes or gene products. If the critical number of daylight hours is not reached, the genes will not be activated, and sexual behavior will not increase.
Each neuron making up the intricate networks and circuits throughout the cerebrum (about 80 percent of the human brain) has protein receptors (chemoreceptors) that respond to specific signaling molecules. The production of the receptors and signaling molecules used for any type of brain activity is directly tied to genes. A slightly different gene may...
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Eugenics (Genetics & Inherited Conditions)
Eugenics is the categorization of a specific human behavior to an underlying genetic cause. People inherit specific genes to build specific pathways that allow them to respond in certain ways to environmental input. With variations possible—from the gene-to-gene regulators to the final cellular response—it is virtually impossible to disconnect the nature-versus-nurture tie that ultimately controls human behavior. Genes are simply the tools by which the environment shapes and reshapes human behavior. There is a direct correlation between gene and protein: Change the gene, change the protein. However, there is no direct correlation between gene and behavior: Changing the gene does not necessarily change the behavior. Behavior is a multifaceted, complex response to environmental influences that is only partially related to genetic makeup. Most studies conducted on humans based on twin and other relative data suggest that most behavioral characteristics have between a 30 and 70 percent genetic basis, leaving considerable room for environmental influence.
Another important fact is that almost no behaviors are controlled by a single gene locus, and the more complex the behavior, the more likely that it is controlled by several to many genes. Hence, not only do environmental effects cloud the picture, but each gene involved in more complex behavioral traits represents just a small part of the genetic basis for the trait as well. The...
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Single-Gene Behavioral Traits (Genetics & Inherited Conditions)
Although behavioral traits controlled by a single gene have been identified, they probably require interaction with other genes in order to produce the specific characteristics of the behavior. On top of this are laid environmental effects. The most dramatic case of a single gene that controls a complex behavior was the discovery in the early twenty-first century of a gene that controls honeybee social status. This same gene is found in fruit flies and affects how actively fruit flies seek food. Bees with a more actively expressed form of the gene (called the for gene) were much more likely to forage than bees with a less active for gene. Not surprisingly, the for gene produces a protein that acts as a cell-signaling molecule.
In humans, only a few behavioral traits are clearly controlled by a single gene. The best examples are Huntington’s disease (a rare, autosomal dominant gene), early-onset Alzheimer’s disease (also a rare, autosomal dominant gene), and fragile X syndrome (actually involves two genes). The remaining traits, as far as has been determined to date, probably represent multigene traits where one primary QTL has been identified as primarily responsible.
Several genes were identified, beginning in the late 1980’s, with possible direct behavioral links. Genes have been implicated in such behaviors as anxiety, depression, hostility, and impulsiveness. One such...
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Multiple-Gene Behavioral Traits (Genetics & Inherited Conditions)
Geneticists concede that for many behavioral traits it may never be possible to sort out the details of the underlying genetic causes. Still, theories abound and researchers continue to speculate. Some genes may play such a minor role that the search for some QTLs will be fruitless. Nevertheless, geneticists have been able to discover QTLs for some important behavioral traits, and the heritability of a number of traits has been determined. The better data available from the Human Genome Project have spawned the rapidly growing field of behavioral genomics, with its emphasis on identifying the specific genetic mechanisms involved in the determination of behavior.
Nonetheless, the quality of the environment matters in most cases. A practical example of this is intelligence or IQ, which is thought by many experts to involve both environmental and genetic influences, given individual abilities to adapt to social stressors. Successful adaptation requires personal coping but may also require either altering the quality of the present surroundings or locating another environment altogether. Such intentional coping also requires a number of mental processes, including sensation, perception, memory, reasoning, learning, and problem solving. The primary thrust is to avoid labeling human mental functioning as strictly nature or nurture, but rather as a selective combination of multiple adaptive processes employed for...
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The Future of Behavioral Genetics (Genetics & Inherited Conditions)
Researchers continue to actively investigate the potential links between behavior and genetics in human functioning. Even when such links are found, however, the degree to which a particular gene is involved and the amount of variation among humans will likely be hard to uncover. The Human Genome Project has greatly accelerated interest in and the search for the genetic bases of behavior, yet with these new data have come an even clearer realization of the complexities of the interplay between genes and behavior. If nothing else, the future should hold more precise answers to the long-standing questions about what makes human beings who they are. The consensus among geneticists today is that behavior is determined neither solely by genes nor solely by the environment. To this end, further research should attempt to make the relative contributions of genes and environment more understandable.
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Further Reading (Genetics & Inherited Conditions)
Avital, Eytan, and Eva Jablonka. Animal Traditions: Behavioural Inheritance in Evolution. New York: Cambridge University Press, 2000. Broadens the evolutionary approach to behavior by arguing that the transfer of learned information across generations is indispensable.
Badcock, C. R. Evolutionary Psychology: A Critical Introduction. Malden, Mass.: Polity Press in association with Blackwell, 2000. An introductory text that addresses such topics as selection and adaptation, survival of the fittest, the benefits and costs of brain evolution, psychological conflict between parent and child, language, and development and conflict.
Benjamin, Jonathan, Richard P. Ebstein, and Robert H. Belmaker, eds. Molecular Genetics and the Human Personality. Washington, D.C.: American Psychiatric Association, 2002. Provides a comprehensive overview of the genetic basis for human personality. Eighteen chapters, each of which ends with a reference section. Index.
Briley, Mike, and Fridolin Sulser, eds. Molecular Genetics of Mental Disorders: The Place of Molecular Genetics in Basic Mechanisms and Clinical Applications in Mental Disorders. Malden, Mass.: Blackwell, 2001. Explores the role of molecular genetics in the understanding of mental disorders and how molecular genetics might help in the development of new drugs for mental illness. Illustrations.
Burnham, Terry, and Jay Phelan....
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Web Sites of Interest (Genetics & Inherited Conditions)
Behavior Genetics Association (BGA). http:www.bga.org. Devoted to the scientific study of the interrelationship of genetic mechanisms and behavior.
Human Genome Project Information, Behavioral Genetics. http://www.ornl.gov/sci/techresources/Human _Genome/elsi/behavior.shtml. This site includes information on the basics of behavioral genetics and links to related resources.
Institute for Behavioral Genetics (IBG). http://ibgwww.colorado.edu. Founded in 1967, the IBG is a research unit of the University of Colorado dedicated to researching the genetic and environmental bases of human behavior.
National Institute of Mental Health, Center for Genetic Studies. http://zork.wustl.edu/nimh. A technical site on the collecting of clinical data to help determine the possible genetic bases of certain mental disorders.
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