Genes are the basic physical and functional unit of heredity. They are segments of DNA that contain instructions for building one or more molecules that help the body work. Genes are passed from parents to offspring and contain the information needed to specify physical and biological traits. 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 protein-coding genes. However, many genes do not code for proteins. 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. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

Scientists keep track of genes by giving them unique names. Because gene names can be long, genes are also assigned symbols, which are short combinations of letters (and sometimes numbers) that represent an abbreviated version of the gene name. For example, a gene on chromosome 7 that has been associated with cystic fibrosis is called the cystic fibrosis transmembrane conductance regulator; its symbol is CFTR.

Genes in general direct specific processes in the body by coding for proteins. A genes primary function can be broken down into four general categories:

• Genes can encode proteins to build something in the body. This can be anything from muscle to bones and cartilage and any other of our internal structures.
• Genes can encode proteins that help direct the body to do something such as muscles flexing or hair growing.
• Genes encode proteins that indirectly support bodily functions. This could be by helping the immune system to respond to injury or to assist blood flow through our circulatory system.
• Genes directly influence our physical characteristics or traits like physical quirks, hair color, eye color, skin tone, and more.

Each unique combination of genetic code is responsible for everything in our bodies, from internal operations to the way we look. With 20,000 to 30,000 genes in the human body, understanding the function of each gene and even understanding how different genes work together continues to be a topic of research.