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What is a gene?
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Basics

What is a gene?

A gene is the basic physical and functional unit of heredity, consisting of DNA sequences that provide instructions for the body to function.

Overview

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, which are needed for the body to function. However, many genes do not code for proteins; instead, they help control other genes.

Genetic Information and Variation

The information in DNA is encoded in genetic building blocks called base pairs. In humans, genes vary in size from a few hundred DNA base pairs to more than 2 million base pairs. The human genome contains about 19,900 genes used to produce proteins.

Typically, people have 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. Forms of the same gene with small differences in their sequence of DNA bases are called alleles. These small differences contribute to each person’s unique physical features.

Naming Conventions

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, the gene associated with cystic fibrosis is called the cystic fibrosis transmembrane conductance regulator, and its symbol is CFTR.

Frequently Asked Questions

VUS indicates that a change in the DNA was found, but current scientific evidence is insufficient to determine whether it is harmless or disease‑causing. Doctors may recommend family testing, periodic re‑evaluation of the variant as new data emerge, or monitoring for related health issues.
Not necessarily. Most genes exist in two copies (alleles). Some diseases are autosomal dominant—one altered allele can cause disease—while others are autosomal recessive—both alleles must be altered. Additionally, penetrance (the chance a gene causes disease) and environmental factors influence whether a person actually develops symptoms.
sample of blood, saliva, or cheek cells is collected and DNA is extracted. The laboratory then uses techniques such as polymerase chain reaction (PCR) to amplify the gene of interest, followed by Sanger sequencing or next‑generation sequencing to read the DNA letters. The results are compared to the reference sequence to identify any mutations.
Yes. Epigenetic mechanisms such as DNA methylation, histone modification, and non‑coding RNA can turn genes on or off. Lifestyle factors (diet, exercise, smoking) and certain medications (e.g., HDAC inhibitors) can modify these epigenetic marks, potentially affecting gene expression, though the impact varies widely among individuals.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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