Simply, an individual’s cells read genetic information in our DNA like words in a book, although DNA’s “alphabet” is a mere four letters—Adenine (expressed as “A”), Guanine (G), Cytosine (C), and Thymine (T). These letters combine to form an individual’s unique genotype, its total set of genes—but organisms are more than just a sequence of letters. In fact, identical twins, with almost the exact same genotype, may look and act very different from each other. An individual’s phenotype is its observable characteristics, and the result of a lifetime of environmental influences acting on his or her genotype. For example, prolonged exposure to the sun makes some people tan, while others burn; how you react to the sun is a result of your genotype. Yet nobody is born tanned or sunburned. Rather, ultraviolet radiation from the sun activates your genes to produce pigments that will darken skin.