Complex Genetics

New techniques allow behavior geneticists to ask not just whether a behavior is under genetic influence, but also what specific genes are involved. To identify genes involved in behavior, investigators use genetic markers— stretches of DNA that differ among individuals. One can either use genetic markers that are evenly spaced on all chromosomes, to search for genes influencing the behavior that are located anywhere in the genome (called genomic screening), or one can test markers at a specific gene believed to be, on theoretical grounds, involved in the behavior (called the candidate gene approach).

The idea behind these analyses is that if a particular gene is involved in the behavior, then people who are more alike with respect to the behavior will be more likely to share the same stretch of DNA that is at or near the gene. The difficulty in searching for genes involved in behavior is that there is no one-to-one correspondence between carrying a particular gene and exhibiting a particular behavior. There are no genes for behavior; there are only genes that influence behavior. Any particular behavior is a complex trait that involves more than one gene and is influenced by the environment as well.

For example, having a particular gene may make a person more likely to have problems with alcohol, but it does not determine whether or not the person will be an alcoholic. Some individuals will carry genes predisposing them to alcohol abuse but will never exhibit any problems, because they choose to abstain from alcohol. Other individuals will exhibit obvious alcohol problems, but will not carry the particular genes known to be involved.

This is because a large number of genes are risk-relevant for use and abuse of alcohol, and each has only a very small effect. Different genes may be acting in different individuals. And genes interact with each other and with the environment. Thus, individual outcomes result from a complex and ill-understood mixture of both genetic and environmental risk factors. That very complexity creates the diverse nature of human behavior. Indeed, it is what makes us uniquely human, but it also makes finding genes involved in human behavior extraordinarily difficult.

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