a defective or abnormal gene or set of genes, a malfunction can occur during combination of maternal and paternal DNA, or exposure to an outside substance or condition can occur after conception that alters the genes in the fetus. Physical and social environments (e.g., family, community, school, culture) interact with and influence these biological processes.
Classically, genes have been considered to be the “instructions” for building proteins, although it is clear now that they have other functions as well. A gene may affect health as a result of the interaction of its protein product with another aspect of a child’s biology. The combinations of these interactions may result in an enhanced, worsened, or inconsequential change in health status. For example, sometimes an alteration in the gene (i.e., mutation) is identified due to the presence of a particular disease state, or it can be deduced that an individual with the mutation has a high probability of developing a particular disease. Understanding the biological pathway of the disease and its interactions with other biological processes facilitates treatment options by modifying the causal path. In the case of Huntington’s disease, for example, the disorder appears to be in part mediated by glutamate excitotoxicity; giving patients a substance that blocks this effect (glutamate receptor antagonists) interrupts this pathway and may retard the manifestation of the disease (Ferrante et al., 2002). The influence of genes on health always exists in an environmental context; in the next sections we describe how genes affect behavior and the physical and social environments.
Genes and Behavior. That genes affect behavior has been amply demonstrated in honeybees (Ben-Shahar et al., 2002) and higher animals (Ruby et al., 2002; Chester et al., 2003; Hendricks et al., 2003). Examples in humans are being rapidly discovered, including genes that influence the relationship between exposure to trauma and susceptibility to posttraumatic stress disorder (Stein et al., 2002), genetic polymorphisms that protect against alcoholism (Wall, Carr, and Ehlers, 2003), mutations that result in sleep disorders (Wijnen et al., 2002), and several genes that are associated with simple phobias (Gelernter et al., 2003).
Genes and the Physical Environment. The physical environment includes ubiquitous agents (e.g., ultraviolet light, amino acids and sugars in the diet, noise, speech) and somewhat less universally encountered ones (e.g., loud music, medications, pollutants). Some genes result in poor outcomes following common environmental exposures (e.g., phenylketonuria with phenylalanine, galactosemia with galactose, xeroderma pigmentosa with ultraviolet light exposures). Individuals with these genotypes are likely to be affected by the disease because they have a high chance of being exposed to the physical environmental agent. In some cases, the physical environment can be modified to improve outcomes