(e.g., a phenylalanine-free or galactose-free diet may improve IQ, no sunlight or ultraviolet exposure may reduce skin cancers). There are also gene alterations resulting from uncommon physical environmental exposures that affect health. Examples include a higher risk of lung cancer in individuals lacking glutathione S-transferase mu who smoke (Perera et al., 2002) and noise-induced hearing loss in some individuals exposed to high levels of noise—the gene or genes in this case are just being discovered (Kozel et al., 2002). These genes are known as susceptibility genes. An example of a positive susceptibility gene to a physical environmental agent may be that for perfect pitch. It appears that individuals with exposure to music and a family history of perfect pitch are more likely to acquire perfect pitch (Alfred, 2000).

Genes and the Social Environment. The identification of genes that confer susceptibility to adverse or beneficial responses following exposure to diverse social environments has only just begun. In one study, abused children with a genotype conferring high levels of neurotransmitter-metabolizing enzyme monoamine oxidase A expression were less likely to develop antisocial problems in adulthood (Caspi et al., 2002). The risk of developing alcohol abuse or dependence also appears to have both a genetic susceptibility and a family influence (Macciardi et al., 1999; McGue et al., 2001), as does the risk for relapse and poor outcomes with schizophrenia (Campbell, 2001).

Characteristics of Gene-Environment Interactions

The expression of certain genetic characteristics depends on the environment in which they occur. Thus, gene expressions that lead to a disease in one context may not lead to a disease, or may result in a different disease, in another context (Holtzman, 2002). Inheriting a single copy of the hemoglobin S gene makes an individual resistant to malaria (Aidoo et al., 2002). However, inheriting two such genes gives the individual sickle cell anemia, a severe disease. Outside of malaria-endemic areas, sickle cell trait, the inheritance of one copy of hemoglobin S, has no known adaptive benefit and may be maladaptive. A single cystic fibrosis gene has been postulated to be protective against diarrheal diseases such as cholera, conferring a survival advantage to individuals who carry one copy of the gene (Rodman and Zamudio, 1991). However, individuals with two such genes have cystic fibrosis, a severe disorder with altered pulmonary and gastrointestinal function. Other examples of genes with positive influence also exist in given environments. The gene or genes that confer protection from cancer (Gonzalez et al., 2002; Reszka and Wasowicz, 2002) have been described.

Genes may confer susceptibility only during a specific span of time, referred to as a critical period. For example, 20 percent of children are extremely sensitive to thalidomide during a critical 15-day period from day 20 to day 35 of gestation, although the gene or genes responsible for this enhanced sensitivity have not yet



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