What is a gene?

A gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins. 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. The Human Genome Project estimated that humans have between 20,000 and 25,000 genes.

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 are made up of DNA. Each chromosome contains many genes.

Credit: U.S. National Library of Medicine

Topics in the Cells and DNA chapter

Other chapters in Help Me Understand Genetics

Genetics Home Reference provides consumer-friendly gene summaries that include an explanation of each gene's normal function and how mutations in the gene cause particular genetic conditions.

More information about how genetic conditions and genes are named is also available from Genetics Home Reference.

The Centre for Genetics Education offers a fact sheet that introduces genes and chromosomes.

The Tech Museum of Innovation at Stanford University describes genes and how they were discovered.

The Virtual Genetics Education Centre, created by the University of Leicester, offers additional information on DNA, genes, and chromosomes.