Behavior Genetics

Behavior genetics

The study of the hereditary factors of behavior. Charles Darwin, who originated the theory that natural selection is the basis of biological evolution, was persuaded by Francis Galton that the principles of natural selection applied to behavior as well as physical characteristics. Members of a species vary in the expression of certain behaviors because of variations in their genes, and these behaviors have survival value in some environments. One example of such a behavior is curiosity—some organisms are more curious than others, and in some settings curiosity is advantageous for survival. The science of behavior genetics is an extension of these ideas and seeks (1) to determine to what extent the variation of a trait in a population (the extent of individual differences) is due to genetic processes, to what extent it is due to environmental variation, and to what extent it is due to joint functions of these factors (heredity-environment interactions and correlations); and (2) to identify the genetic architecture (genotypes) that underlies behavior.

Traditionally, some of the clearest and most indisputable evidence for a hereditary influence on behavior comes from selective-breeding experiments with animals. Behavior genetic research has utilized bacteria, paramecia, nematodes, fruit flies, moths, houseflies, mosquitoes, wasps, bees, crickets, fishes, geese, cows, dogs, and numerous other organisms. Breeding of these organisms allows genetically useful types of relationships, such as half-sibs, to be produced easily. Artificial selection (selective breeding) can be used to obtain a population that scores high or low on specific traits. Inbred strains of many animals, particularly rodents, are readily available, and the study of various types of crosses among them can provide a wealth of information. An experimental design using the recombinant inbred-strain method shows great promise for isolating single-gene effects. This procedure derives several inbred strains from the F₂ generation (grandchildren) produced by a cross between two initial inbred strains. Since it is possible to exert a great deal of control over the rearing environments, the experimenter can manipulate both heredity and environment.

Other work has focused on the effects of the environment and genotype-environment interactions. For example, experiments with mice have shown that, with respect to several learning tasks, early environmental-enrichment effects and maternal effects were quite small, relative to the amount of normal genetic variation found in the strains of mice tested. Only a few genotype-environment interactions were found. Still other work has shown that early experiences affect later behavior patterns for some strains but not others (a genotype-environment interaction).

An increasing role for animals in genetic research is to provide models of human genetic diseases, many of which have behavioral features. Such animal models may occur naturally or may be engineered in the laboratory. Animal models are available for many neurobehavioral disorders, including narcolepsy, various epilepsies, and alcoholism. The availability of animal models allows researchers to obtain information about the development of genetic disorders and the effects of different environments on this development, as well as to explore treatment options. While it is not always prudent or desirable to generalize from animal results to humans, it is assumed that basic genetic systems work in similar ways across organisms, and it is likely that these types of animal studies will play a key role in elucidating the ways in which environment influences phenotypic variation. With advances in genetic technology, it is possible to observe genetic variation more directly by locating, identifying, and characterizing genes themselves.

The effects of a single gene on behavior have been most extensively studied in the domain of mental retardation. Research has shown that there are a large number of metabolic pathways which have defects due to a single gene. Over 100 of these defects influence mental ability. One such single-gene defect is classic phenylketonuria (PKU), an autosomal recessive disorder, which also illustrates the role that environment can play in the expression of a trait. Individuals who are homozygous (having two copies of the PKU allele) are unable to make the enzyme phenylalanine hydroxylase, which converts the essential amino acid phenylalanine to tyrosine, a nonessential amino acid. Instead, the excess phenylalanine builds up in the blood and is converted into phenylpyruvic acid, which is toxic to the developing nervous system in large amounts. The main effect of untreated PKU is severe mental retardation, along with a distinctive odor, light pigmentation, unusual gait and posture, and seizures. Many untreated individuals with PKU show fearfulness, irritability, and violent outbursts of temper. See Mental retardation

Every organism develops in a particular environment, and both genes and environment control development. It is, therefore, not possible to state that a particular behavioral trait is either genetic or environmental in origin. It is possible, however, to investigate the relative contributions of heredity and environment to the variation among individuals in a population. With humans, it is possible to obtain approximate results by measuring the similarity among relatives on the trait of interest. Twins are often used in such studies. One method compares the similarity within pairs of both identical twins and fraternal twins reared together. Identical twins have all their genes in common by descent, since they arise from a single fertilized egg. Fraternal twins arise from two fertilized eggs and so share on average one-half of their genes. If it is assumed that the effects of the shared environments of the two types of twins are equal (a testable assumption), greater resemblance between identical twins than fraternal twins should reflect the proportion of genes they share, and the difference between the correlations of the two twin types should represent about one-half the genetic effect.

A second type of twin study compares not only twins reared together but twins who have been reared apart. The degree of similarity between identical twins reared in the same home would reflect the fact that all their genes are identical and that they share a common family environment. On the other hand, if identical twins can be located who had been adopted by different families chosen at random (an unlikely event, since adopted children tend to be selectively placed), a measure of their degree of similarity would reflect only the effect of their common genes. If it were true that an individual's level on a measure (for example, extroversion) is determined in large part by the characteristics of his or her family and the opportunities that the family makes available to him or her, reared-apart identical twins should be no more alike than pairs of individuals chosen at random. If they do exhibit some degree of similarity, it would reflect genetic effects alone. The existence of even very large genetic effects, however, would in no way imply that the environment was unimportant in the development of the trait; it would simply imply that environment was less important than genes in determining the variation among individuals on the trait in question at the time of measurement. That is, the individuals would differ more because of the genes they carry than because of the particular environments to which they were exposed. In another range of environments, the results might be different. See Twins (human)

Developmental psychologists are finding that differences in children's behavioral phenotypes are due more to their different genotypes than to their different rearing environments, as long as those environments are within a normal range of experiences. Identifying environmental variables from this normal range that have an important effect on the behavioral phenotype may be even more difficult than identifying contributing genes. Advances in theory and new technologies, combined with information from more traditional methodologies, will continue to provide insight into the contributions of genes and environment to behavior.

Behavior Genetics