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Not all inborn characteristics are inherited. Some result from mutations, sudden changes and abnormalities in the structure of genes that occur spontaneously or may be induced by exposure to environmental toxins such as radiation and agricultural chemicals in food (Lewis, 2017). A mutation may involve only one gene or many. It is estimated that as many as one-half of all conceptions include mutated chromosomes (Plomin et al., 2013). Most mutations are fatal—the developing organism often dies very soon after conception, often before the woman knows she is pregnant (Sadler, 2018).

Sometimes mutations are beneficial. This is especially true if the mutation is induced by stressors in the environment and provides an adaptive advantage to the individual. For example, the sickle cell gene (discussed earlier in this chapter) is a mutation that originated in areas where malaria is widespread, such as Africa (Ware et al., 2017). Children who inherited a single sickle cell allele were more resistant to malarial infection and more likely to survive and pass it along to their offspring (Croke et al., 2017; Gong, Parikh, Rosenthal, & Greenhouse, 2013). The sickle cell gene is not helpful in places of the world where malaria is not a risk. The frequency of the gene is decreasing in areas of the world where malaria is uncommon. For example, only 8% of African Americans are carriers, compared with as much as 30% of Black Africans in some African countries (Maakaron & Taher, 2012). Therefore, the developmental implications of genotypes—and mutations—are context specific, posing benefits in some contexts and risks in others. Recent advances in genetic engineering hold important implications for understanding and treating genetic disorders, as discussed in the Lives in Context feature.