Genes are segments of DNA 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 short section of DNA that is made up of a sequence of these bases. Each gene carries instructions that determine a persons features, such as eye color, hair color, and height. Some genes act as instructions to make molecules called proteins, which perform various functions in the body to keep a person healthy. However, many genes do not code for proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. 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.