Gene

A gene is a specific segment of DNA that contains the instructions needed to build a particular protein or functional RNA molecule, making it the basic physical and functional unit of heredity. Genes are passed from parents to offspring and collectively determine many of an organism inherited traits, from eye color to susceptibility to certain diseases, though most complex traits also depend heavily on environmental influences interacting with multiple genes at once. The Human Genome Project, completed in 2003, determined that humans carry approximately 20,000 to 25,000 genes, a number far lower than scientists had originally expected and only modestly higher than the gene count found in much simpler organisms, a finding that surprised many biologists who had assumed that an organism as complex as a human would require substantially more genes than a roundworm or a fruit fly.

Structure and the genetic code

Each gene is composed of a specific sequence of the four chemical bases that make up DNA, commonly abbreviated A, T, C, and G, and the particular order of these bases encodes the information needed to build a protein. Genes are organized along chromosomes, with the specific location of a gene on a chromosome called its locus, and the complete set of genetic material in an organism is known as its genome. Not all DNA within a gene codes for protein directly, since genes also contain regulatory regions that control when and how strongly a gene is activated, as well as, in humans and other eukaryotes, non-coding segments called introns that are removed before the final protein-building instructions are assembled, in a process called splicing that also allows a single gene to produce more than one version of a protein depending on which segments are kept.

From gene to protein

Converting the information in a gene into a functioning protein involves two main steps. First, during transcription, the DNA sequence of a gene is copied into a related molecule called messenger RNA, which carries the genetic instructions out of the cell nucleus. Second, during translation, cellular structures called ribosomes read the messenger RNA sequence and assemble a corresponding chain of amino acids into a protein, following rules collectively known as the genetic code, in which specific three-base sequences called codons each specify a particular amino acid. This flow of information from DNA to RNA to protein, often summarized as the central dogma of molecular biology, explains how a gene static sequence ultimately produces the dynamic molecular machinery that carries out nearly every function in a living cell.

Alleles, mutation, and inheritance

Most genes exist in multiple versions, called alleles, which differ slightly in their DNA sequence and can produce different versions of the resulting trait, such as the alleles that produce different human blood types, where distinct alleles of a single gene determine whether a person is type A, B, AB, or O. Offspring typically inherit one copy of most genes from each parent, and the specific combination of alleles an individual carries, called their genotype, interacts with environmental factors to produce their observable traits, or phenotype. A mutation is any change to a gene DNA sequence, and while many mutations have little or no noticeable effect, some disrupt a gene function enough to cause inherited disease, and others, over the course of evolution, introduce the raw genetic variation that natural selection acts upon. Gregor Mendel, working with pea plants in the 1860s, first deduced the basic rules by which traits pass predictably from one generation to the next, decades before DNA itself was identified as the physical carrier of this information, work that went largely unrecognized during his own lifetime and was only rediscovered and appreciated by other scientists around the turn of the twentieth century.

Genes in medicine and biotechnology

Thousands of inherited diseases, including cystic fibrosis and sickle cell disease, are caused by mutations in a single identifiable gene, and genetic testing can now detect many of these mutations directly, informing decisions about family planning, treatment, and disease risk. Gene therapy attempts to treat disease by introducing, removing, or correcting genetic material within a patient cells, an approach that has moved from experimental research into a small but growing number of approved treatments over the past decade. Genetic engineering techniques allow scientists to transfer genes between organisms entirely, producing genetically modified crops engineered for pest resistance or improved nutrition, and enabling bacteria to manufacture human proteins such as insulin on an industrial scale. Newer gene-editing tools, including CRISPR, have made it dramatically faster and cheaper to modify specific genes with precision, accelerating research across nearly every field of biology and medicine.

FAQs

What is a gene made of?
A gene is a specific segment of DNA, made up of a sequence of four chemical bases, that contains the instructions needed to build a particular protein or functional RNA molecule.

How many genes do humans have?
The Human Genome Project determined that humans carry approximately 20,000 to 25,000 genes, a figure that was lower than scientists had originally expected.

What is the difference between a gene and an allele?
A gene is a specific DNA sequence coding for a trait, while an allele is one of the alternative versions of that gene that can exist within a population, producing variations in the resulting trait.

What is the central dogma of molecular biology?
It describes the flow of genetic information from DNA to messenger RNA through transcription, and from RNA to protein through translation, explaining how a gene sequence ultimately produces a functioning protein.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 2, 2026Researched and drafted with AI assistanceReport an issue