what is a gene in biology

A gene is the basic physical and functional unit of heredity. It is a segment of DNA that contains instructions for building one or more molecules that help the body work. Genes are made up of nucleotides, which are the structural units of a nucleic acid. Most genes code for specific proteins, or segments of proteins, which have differing functions within the body. 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 contains approximately 20,000 to 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. These small differences contribute to each person’s unique physical features. Alleles are forms of the same gene with small differences in their sequence of DNA bases. An individual’s phenotype is determined by the combination of alleles they have.