It might be startling to many that the way they communicate, form relationships, respond to life situations, or simply their usual behavior could be affected by genes to some extent. This field of study that links behavior with genes is the genetics of behavior. Not just the animal behaviors are a product of genes and environment, but human beings too have similar disposition regarding behavioral genetics. It is a field of research about how genes and environments interact through development to shape differences in mood, personality, and intelligence.
Behavior is generally defined as a reaction to stimuli, whether internal or external, that alter an organism’s response to its environment. Animals run, remain still, or counterattack to predators; birds build complex and distinctive nests in response to a combination of internal and external signals; plants bend toward the light; and humans behave in both simple and complex ways guided by their intellect, emotions, and culture. Behavior relies on the expression of an individual’s genotype, which takes place within a hierarchy of environmental settings. Gene expression depends on interactions within the cell, tissue, organ, organism, and finally, the population and the surrounding environment.
Attempts to study the role of heredity in behavior began early in the nineteenth century with Francis Galton, who systematically studied behavior and heredity. Galton, who was a cousin of Charles Darwin, studied his own family, trying to find a pattern of inheritance associated with intelligence. Distinctive cases of genetic influence on behavior had been identified by the early twentieth century. By the 1950s, it became clear that behavioral patterns were the result of the interaction of genetic and environmental factors. Without genes and their environments, there could be no behavior.
Much of the study of behavioral genetics focuses on the development, structure, and function of the nervous system because the nervous system senses the environment, processes the information, and initiates the response that one perceives as behavior. Evidence that some behaviors have a genetic basis comes from several sources: observable species-specific behaviors, such as courtship rituals, and artificial selection, which draws upon individual variations in behavior among organisms in populations that can be used to establish strains with heritable differences in behavior. In humans, twin studies and adoption studies have provided evidence for the role of heredity in behavioral responses.
Different approaches have been used to study the genetic control of behavior, to define the interactions between genotype and environmental factors, and to dissect the pathways leading from genes to a behavioral phenotype. One of these approaches is a behavior-first method. It begins by selecting organisms exhibiting a specific behavior from members of a genetically heterogeneous population. If genetic strains can be established that uniformly express this behavior, and if genetic crosses can transfer the trait to another strain that initially does not exhibit the behavior under study, genetic involvement in the behavior is confirmed.
The second approach is a bottom-up, or gene-first approach, in which mutagenesis followed by screening is used to identify single-gene mutations associated with variant or abnormal behaviors. Analysis of the molecular mechanism of gene expression in these mutant strains often provided a direct explanation of the behavior. A third approach to studying the genetics of behavior is based on methods that allow entire genomes to be scanned to identify genes and markers for genes and genome variations related to specific behaviors.
Genetic components in Human Behavior
The biochemical pathways that control learning and memory in Drosophila are similar to those in other organisms, including mice and humans. However, the genetic control of behavior has proven more difficult to characterize in humans than in other organisms. One of the reasons for this is the complexity of the behavioral aspects of a human, such as intelligence, language, personality, and emotion. These traits are difficult to define objectively and to measure quantitatively, and the influence of environmental factors cause a wide range of variation in individual responses. In each case, the environment is vital in shaping, limiting, or facilitating the final phenotype. In humans, behavior genetics can be studied with three approaches: analysis of single genes with behavioral components, the use of animal models, and genomic methods.
Single Genes and Behavior
Genetics of behavior can be studied through the cause and progression of neurodegenerative diseases associated with genes. Neurodegenerative disorders affect millions of people across the world. These diseases are associated with progressive accumulation of misfolded proteins in intracellular and extracellular spaces, leading to the death of brain cells, with behavioral consequences. Some of the diseases caused by a mutation in single genes are Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis (ALS), and Huntington disease. Transgenic animal models of neurodegenerative diseases caused by single-gene mutations, including AD, ALS, and HD, have been developed to study the molecular events of synthesis, processing, and aggregation of mutant proteins, and the links between protein accumulation, cell death, and behavioral changes. For instance, the behavioral changes and the degeneration in specific brain regions in the transgenic mice parallel the progression of HD in humans.
Complex Behavioral Traits
While behavioral disorders such as HD associated with single-gene could be successfully studied, other behaviors that are polygenic with strong environmental components are more difficult to dissect. For example, schizophrenia, a brain disorder affecting about 1 percent of the population and is an example of a complex genetic disease. Schizophrenia is a collection of mental disorders characterized by avoidance of social contact and by bizarre and sometimes delusional behavior. It is a familial disorder, with relatives of schizophrenics having a much higher incidence of the condition than the general population.
Furthermore, the closer the genetic or biological relationship to an affected individual, the greater is a person’s probability of developing the disorder. Twin studies have helped establish a genetic link to schizophrenia. Schizophrenia is currently regarded as a multifactorial and quantitative trait, influenced by genes, by the environment, and by interactions between the genotype and environmental factors. The evidence from GWAS on schizophrenia indicates that no single gene or allele makes a significant contribution to this disorder. Instead, the results point to the involvement of hundreds of genes that each contributes only a small amount to schizophrenia.
Behavior is a multifactorial trait controlled by genes and environmental factors. Genomic technology has opened new avenues of investigation into the development of the human nervous system and the mechanisms of behavioral disorders. In many ways, researches in this field have led to the conclusion that behavioral responses are mediated by genes, environmental factors, and interactions among genes and the environment.