A simple defi nition of a gene is that it is a segment of DNA that codes for the amino acid sequence of a protein (enzymes, neurotransmitters, hormones, or cell-structure proteins). These proteins do not cause us to behave or feel one way or another; they facilitate our behavior and our feelings and modulate how we respond to the environment. Genes are not automatons pulling strings in our heads and determining the directions of our lives; rather they help us to get there once we decide on the direction, although they do bias us to prefer one direction over the other. They are part of an organism’s machinery for responding and adapting to its environment composed of the sensory, evaluative, production, and motor systems. The afferent nerves sense and transmit information about the state of a person’s internal or external environment to the central nervous system (CNS) to be evaluated. Genes then respond to the information evaluated by the brain by manufacturing the appropriate proteins. Efferent nerves then carry impulses away from the CNS to the motor systems (muscles or glands) that initiate the organism’s overt reactions.