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Genes

A gene is the basic physical and functional unit of heredity: a stretch of DNA that carries information a cell can use. Many genes hold the instructions for making a protein, which the cell reads in two steps — transcription into messenger RNA, then translation into a chain of amino acids — while many others make no protein and instead help control other genes. People usually carry two copies of each gene, one from each parent, and slightly different versions of a gene are called alleles.

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Genes, variants and the human gene count

A gene is the basic physical and functional unit of heredity. Protein-coding genes are expressed by transcription to mRNA and translation on ribosomes, but a substantial share of genes produce no protein and instead regulate other genes. Diploid humans carry two copies of each gene; sequence differences between copies define alleles.

  • Germline (inherited) variants are present in the egg or sperm and so appear in virtually every cell of the child.
  • Somatic variants arise during life — for example from ultraviolet damage or DNA-copying errors during cell division — and cannot be passed on.
  • Most variants do not lead to disease; those that do are uncommon in the general population.
  • The finished 2004 human genome sequence (Build 35) had 2.85 billion nucleotides and about 99% euchromatic coverage.
  • Protein-coding gene estimates moved from 20,000–25,000 at the end of the Human Genome Project to about 19,900 today.
Info: The word 'mutation' is being replaced by 'variant' in clinical genetics because a change in DNA does not necessarily cause disease.
Common misconception: A common slip is to quote the Human Genome Project estimate of 20,000–25,000 protein-coding genes as the final word. Later studies refined the figure to about 19,900, so any gene count should be cited with its source and date.
Full explanation — the complete reference version every reading depth is based on

What a gene is

A gene is a particular stretch of DNA that works as a unit of heredity. The NIH's MedlinePlus Genetics describes it as the basic physical and functional unit of heredity. Some genes are instructions for building a protein; many others never become protein at all and instead help switch other genes on and off. Human genes vary enormously in length, from a few hundred DNA base pairs to more than two million.

  • Genes are made of DNA and sit on chromosomes inside the cell.
  • People typically have two copies of each gene — one inherited from each parent.
  • Versions of the same gene that differ slightly in their DNA sequence are called alleles.
  • Most genes are the same in everyone; less than 1 percent of them differ slightly from person to person.
  • The human genome holds about 19,900 protein-coding genes.

From gene to protein

Using a protein-coding gene is called gene expression, and it happens in two steps. In transcription, the information in the gene's DNA is copied into a related molecule, messenger RNA (mRNA), inside the nucleus. In translation, the mRNA is read in the cytoplasm by a ribosome, three nucleotides at a time. Each three-letter group, a codon, usually stands for one amino acid, and transfer RNA brings the matching amino acids so the protein grows one link at a time until a stop codon is reached. This one-way flow of information — DNA to RNA to protein — is often called the central dogma of molecular biology.

ncoding nucleotides=3×namino acidsn_{\text{coding nucleotides}} = 3 \times n_{\text{amino acids}}

Because each codon is three nucleotides and usually specifies one amino acid, a chain of n amino acids needs 3n coding nucleotides of mRNA, plus three more for the stop codon. (Our own arithmetic from the codon rule.)

Worked example

A small protein has 150 amino acids. Its coding mRNA must contain 3 × 150 = 450 nucleotides to specify those amino acids, and the stop codon adds 3 more, so the ribosome reads 453 nucleotides from the first codon to the end of the stop codon.

Genes are switched on and off

Almost every cell in your body carries the same set of genes, yet a nerve cell and a muscle cell look and behave very differently. The reason is gene regulation: each cell turns on only a fraction of its genes at any moment and keeps the rest switched off, and different cell types switch on different sets during development.

Common misconception: A common misconception is that every gene is always 'on'. In reality each cell expresses only a fraction of its genes at any given time; which genes are on is what makes a liver cell different from a skin cell.

When genes change

A permanent change in a gene's DNA sequence is called a gene variant (the older word is mutation). Variants inherited from a parent are in virtually every cell of the body. Somatic variants appear during a person's life — for example after damage from ultraviolet light or a copying error when a cell divides — and exist only in some cells, so they cannot be passed on to children.

Common misconception: It is easy to assume that a gene variant always means a disease. Most variants do not lead to disease at all — small differences in genes are a large part of why people differ in harmless ways.

Where this connects

Genes build directly on DNA, and they are the units passed from parent to offspring in inheritance. Differences between alleles are the raw material that natural selection acts on, and the immune system's huge range of antibodies is produced by rearranging and mutating gene segments.

How we know

Thomas Hunt Morgan's work on the role of chromosomes in heredity earned the 1933 Nobel Prize in Physiology or Medicine. Between 1990 and 2003 the international Human Genome Project sequenced the human genome, and the finished sequence published in 2004 covered about 99% of the gene-rich (euchromatic) part of the genome. That work estimated 20,000–25,000 protein-coding genes; later studies refined the figure to about 19,900.

Interactive 3D scene

DNA Structure Explorer

Explore the DNA double helix in an interactive 3D scene showing real Watson-Crick base pairing and B-DNA structural geometry, with base-pair highlighting and a sequence view.

Explore the DNA double helix in 3D — genes are particular stretches of this molecule.

Assumptions and limits

  • 'Two copies of each gene' is the typical case: one copy comes from each parent, but a variant can be present in only some of a person's cells.
  • The three-nucleotides-per-amino-acid rule applies to the coding part of an mRNA; a codon usually, not always, stands for one particular amino acid.
  • A gene count is a best estimate tied to how well the genome has been sequenced and annotated, so it should always be quoted with its source and date.

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Content status: published 1 October 2026.

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