A gene is the basic physical and functional unit of heredity. It is a specific sequence of nucleotides in DNA or RNA that is located usually on a chromosome and that is the functional unit of inheritance controlling the transmission and expression of one or more traits by specifying the structure of a particular polypeptide and especially a protein or controlling the function of other genetic material. Genes carry the information that determines physical traits, such as hair color, eye color, and height, and many other characteristics that are passed on to offspring from their parents. Most genes code for specific proteins, or segments of proteins, which have differing functions within the body. Humans have approximately 20,000 to 25,000 genes. Every person has two copies of each gene, one inherited from each parent. Genes are found on tiny structures called chromosomes, which are found inside cells. Chromosomes come in matching sets of two (or pairs), and there are hundreds, sometimes thousands, of genes in just one chromosome.