Genes are the basic physical and functional unit of heredity. They are segments of DNA (or RNA in some viruses) that contain instructions for building the molecules that make the body work. DNA is made up of millions of small chemicals called bases, which come in four types: A, C, T, and G. A gene is a section of DNA made up of a sequence of As, Cs, Ts, and Gs. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. Each gene carries instructions that determine a persons features, such as eye color, hair color, and height. Genes are found in every one of the 37 trillion cells that make up the human body. Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. These small differences contribute to each persons unique physical features. Scientists keep track of genes by giving them unique names and symbols, which are short combinations of letters (and sometimes numbers) that represent an abbreviated version of the gene name.