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ALL BEHAVIORS ARE SHAPED BY THE interplay of genes and the environment. The most stereotypic behaviors of simple animals are influenced by the environment, while the highly evolved behaviors of humans are constrained by innate properties specified by genes. Genes do not control behavior directly, but the RNAs and proteins encoded by genes act at different times and at many levels to affect the brain. Genes specify the developmental programs that assemble the brain and are essential to the properties of neurons, glia, and synapses that allow neuronal circuits to function. Genes that are stably inherited over generations create the machinery by which new experiences can change the brain during learning.

In this chapter, we ask how genes contribute to behavior. We begin with an overview of the evidence that genes do influence behavior, and then review basic principles of molecular biology and genetic transmission. We then provide examples of the way that genetic influences on behavior have been documented. A deep understanding of the ways that genes regulate behavior has emerged from studies of worms, flies, and mice, animals whose genomes are accessible to experimental manipulation. Many persuasive links between genes and human behavior have emerged from the analysis of human brain development and function. Despite the formidable challenges inherent in studying complex traits in humans, recent progress has begun to reveal the genetic risk factors in neurodevelopmental and psychiatric syndromes such as autism, schizophrenia, and bipolar disorder, offering another important avenue to clarify the relationship between genes, brain, and behavior.

An Understanding of Molecular Genetics and Heritability Is Essential to the Study of Human Behavior

Many human psychiatric disorders and neurological diseases have a genetic component. The relatives of a patient are more likely than the general population to have the disease. The extent to which genetic factors account for traits in a population is called heritability. The strongest case for heritability is based on twin studies, first used by Francis Galton in 1883. Identical twins develop from a single fertilized egg that splits into two soon after fertilization; such monozygotic twins share all genes. In contrast, fraternal twins develop from two different fertilized eggs; these dizygotic twins, like normal siblings, share on average half their genetic information. Systematic comparisons over many years have shown that identical twins tend to be more similar (concordant) for neurological and psychiatric traits than fraternal twins, providing evidence of a heritable component of these traits (Figure 2–1A).

Figure 2–1

Familial risk of psychiatric disorders provides evidence of heritability.

A. Correlations between monozygotic twins for psychiatric disorders are considerably greater than those between dizygotic twins. Monozygotic twins share nearly all genes and have a high (but not 100%) risk of sharing the disease state. Dizygotic twins share 50% of their genetic material. A score of zero represents no correlation (the average result for two random people), whereas ...

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