A gene is the basic physical and functional unit of heredity. Genes,
which are made up of DNA, act as instructions to make molecules called
proteins. In humans, genes vary in size from a few hundred DNA bases to
more than 2 million bases. The Human Genome Project has 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.
- NLM National LIbrary of Medicine