Behavioral genetics is a multidisciplinary field that studies both the genetic and environmental contributions to variations in behavior. Because genes cannot exist without an environment to act upon, behavioral genetics must consider the unique experiences of an organism. Behavioral geneticists seek to understand how genes influence behavior. Etiology, the study of the cause of disease, is associated with behavioral genetics because many diseases such as Huntington disease have an underlying genetic basis that manifests particular behaviors in affected individuals. Other examples of inherited diseases that express behavioral traits such as mental retardation as part of their phenotype include Down syndrome (trisomy 21) and phenylketonuria (PKU).
Behavioral genetics include studies of twins, related and unrelated family members, adoptees, and individuals with genetic abnormalities. Familial studies are used to determine whether or not a particular trait is inherited by the relative risk for family members inheriting that trait. For example, research suggests that the relative risk for someone to develop schizophrenia throughout his or her lifetime is about one percent. However, siblings and children of schizophrenics are 10 and 13 times more likely to develop schizophrenia, respectively. Other family members have a two percent to three percent risk for developing schizophrenia; thus, confirming that schizophrenia is a familial trait. Twin and adoption studies are more accurate in helping researchers determine whether behaviors are due to shared genes or shared environments. Researchers look for genes that are associated with particular behaviors. For example, removing a particular gene may evoke a behavioral response whereas if the gene remains in place it evokes another behavior. This approach has served to associate specific genes to mental illnesses such as schizophrenia and bipolar disorder. However, because of the unpredictable onset and variability in behavior, it is believed that the environment plays a substantial role in mental illnesses.
Behavior can be defined as any observable action or conduct of an organism that is due to the integration of the central nervous system. Understanding molecular biology provides an explanation of the physiological process that enables genetics to influence behavior. The DNA that makes up genes, codes for proteins in the process of transcription and translation. Proteins are responsible for the makeup of the physical and chemical characteristics of cells. These cells that are the constituents of tissues and organs are capable of synthesizing chemicals. Neurotransmitters are chemicals utilized in the brain for communication of one nerve cell to another. In order to function in communication, neurotransmitters must be released from the end of one neuron and bind to protein receptors of an adjacent neuron. The binding of neurotransmitters to protein receptors causes either an excitatory or inhibitory response telling our neurons to continue or stop communication, respectively. In either case, the response of nerve cells has a direct effect on our thoughts and actions occurring in the brain. In 1991, Adams, et al. concluded that 30% of the approximately 50,000 human genes are expressed in the brain, providing evidence that behavior is at least in part influenced by genetics.
Many behavior altering diseases and disorders are due to abnormal excitatory and inhibitory neuronal responses. Irregular responses are a direct result of excessive or inadequate amounts of neurotransmitters binding to receptor proteins. Several factors can influence the binding of neurotransmitters to protein receptors, such as an abundant amount of neurotransmitters released from a nerve cell or an overabundance of protein receptors available to bind the neurotransmitters, both resulting in an excitatory response. In addition, an excitatory response may occur if not enough enzymes are available to break down the neurotransmitters, allowing it to remain bound to the receptor protein for longer periods of time. Inhibitory responses can be due to lack of neurotransmitters available to bind to the receptor proteins, too many enzymes to break down the neurotransmitters too soon, and not enough receptor proteins available for binding. Consequently, it is the genes that are responsible for determining the amount of neurotransmitters, enzymes, and protein receptors produced in the brain. The resultant integration of these neuronal components produces the observable actions of organisms called behavior.
PKU is an inherited metabolic disorder resulting in a defective gene that is unable to code for an enzyme necessary for amino acid metabolism. As a result, phenylalanine builds up in the blood and nervous system and cannot be used in the synthesis of norepinephrine, epinephrine, dopamine, and melanin. These neurotransmitters are essential for normal brain functions; consequently, brain damage and mental retardation set in and clearly affect behavior. However, by reducing dietary intake of phenylalanine and taking medication, the buildup of the amino acid can be reduced providing an example of how environmental control can influence the behavioral effects of a genetic disorder.
One problem that plagues geneticists is how to describe the phenotype; in the case of behavioral genetics, it is how to describe the behavior. The underlying difficulty lies in that variant behavior exists on a continuous spectrum and attempting to measure behavior is subject to human error. Moreover, a common misconception exists stating all behavior results as an all or none effect from genes; in other words an individual that has a gene will inevitably express the trait for that gene. Geneticists use the term penetrance to clarify this misconception. Penetrance indicates the probability that the trait will be expressed if an individual is carrying the gene for that trait. Some disorders such as Huntington disease are easily discernible as a disease clearly marked by behavioral genetics. Huntington disease is a disorder that is completely penetrant, that is to say that everyone who carries the gene will express the behavior of the gene. The disease is characterized by changes in behavior such as dementia, difficulty swallowing, and difficulty speaking that begin in adulthood and are due to neurological degeneration that is ultimately fatal.
Dyslexia, a learning disorder characterized by difficulties in reading, is an example of a disorder with incomplete penetrance. People who carry the gene for dyslexia do not always express the phenotype; others express the gene at varying degrees. Incomplete penetrance can be due to environmental factors or the interaction of other genes. Research suggests that genes contribute between 40%-60% of an individual's behavioral variance. Thus, disorders exhibiting incomplete penetrance may be characterized by behaviors that exist on a continuous spectrum without exact evidence to determine how much genetic and environmental factors affect the behavior.
Technological advances in molecular biology have provided techniques for analyzing DNA to help solve some of the mysteries in behavioral genetics. Linkage studies have become a popular technique for attempts at identifying behavioral genes. Researchers who want to determine a gene for a particular disorder find an affected individual. Next, researchers look at DNA sequences that the person with the disorder shares with his or her family members or other individuals who also express the behavior. The purpose is to find DNA markers close to the gene and then identify the gene itself. The DNA marker need not be the exact gene because the probability of separation of the DNA marker and gene during genetic recombination is minute. Once a gene is identified, it is subject to further analysis in order to determine its behavioral effect. The problem with linkage studies remains in the fact that most behaviors are believed to be the result of multiple genes. Moreover, it is hypothesized that the frequencies of alleles in a complex array of genes may manifest some form of a behavior while a different combination elicits another form of the same behavior.
Sir Francis Galton was the first to systematically study the effects of genetics on behavior. He studied behavioral correlations within families and utilized twins to determine how variances in intelligence and stature could be influenced by genetics. Robert Tyron began studies on behavioral genetics in the 1930s at the University of California, Berkeley. Tyron began his studies selectively mating rats from a single group in order to breed specific behavioral traits in two lines of rats. One line of rats was bred to successfully run a maze with virtually no errors while the other line of rats was bred to perform poorly. After several generations of selective mating, Tyron was able to conclude that behavioral traits are controlled by genetic variation. In another study in 1934, Calvin Hall concluded that genetics was an influential factor in the emotional behavior of mice. Through 30 generations of selective breeding, Hall was able to breed one line of mice to exhibit confidence and explore its new surroundings when trapped in a brightly lit space. The other line of mice appeared terrified and would defecate when exposed to the same conditions.
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