What Is a Gene? Definition, Function, DNA & Alleles Explained

What Is a Gene?

A gene is a functional unit of heredity encoded in DNA. In biological terms, a gene is a DNA sequence that contains information used to produce a functional product, such as a protein or functional RNA molecule.

Genes are transmitted from parents to offspring and contribute to inherited biological characteristics. They influence processes ranging from cell metabolism and development to blood-group characteristics and susceptibility to certain genetic disorders.

In humans, genes are located primarily on chromosomes in the cell nucleus, although mitochondrial DNA also contains a small number of genes.

=> A gene is a specific sequence of DNA containing biological instructions that a cell can use to make a functional product.

In humans, genes are located primarily on chromosomes in the cell nucleus, although mitochondrial DNA also contains a small number of genes.

=> A gene is a specific sequence of DNA containing biological instructions that a cell can use to make a functional product.

Blog image — Gene - an overview | ScienceDirect

What Is DNA?

DNA, or deoxyribonucleic acid, is the molecule that stores hereditary information in most living organisms.

DNA consists of two complementary strands arranged into a double helix. Its genetic information is encoded through four nucleotide bases:

A - Adenine
T - Thymine
C - Cytosine
G - Guanine

The order of these bases forms the DNA sequence.

Genes are specific functional regions within DNA. Therefore, DNA and genes are not synonymous: DNA is the genetic material, while a gene is a defined DNA sequence with a biological function.

DNA, or deoxyribonucleic acid, is the molecule that stores hereditary information in most living organisms.

DNA consists of two complementary strands arranged into a double helix. Its genetic information is encoded through four nucleotide bases:

A - Adenine
T - Thymine
C - Cytosine
G - Guanine

The order of these bases forms the DNA sequence.

Genes are specific functional regions within DNA. Therefore, DNA and genes are not synonymous: DNA is the genetic material, while a gene is a defined DNA sequence with a biological function.

What Is the Difference Between DNA, a Gene and a Chromosome?

These terms describe related but different levels of genetic organization.
Term	Definition
Item one	Description
DNA	The molecule that stores genetic information.
Gene	A functional DNA sequence that contributes to the production of a functional RNA or protein product.
Chromosome	A highly organized DNA–protein structure containing many genes and other DNA sequences.
Genome	The complete genetic material of an organism.
Allele	An alternative form or sequence variant of a gene at a particular locus.

A useful hierarchy is :

Cell → Nucleus → Chromosomes → DNA → Genes

However, this simplified model has exceptions. For example, mitochondria contain their own DNA and genes outside the nucleus.

How Do Genes Work?

What Are Amino Acids and How Are They Related to Genes?

Amino acids are the molecular building blocks of proteins.

For protein-coding genes, the nucleotide sequence ultimately influences the sequence of amino acids incorporated into a protein.

The relationship can be summarized as:

Gene → transcription → mRNA → translation → amino-acid sequence → protein

Changes in a gene's DNA sequence can sometimes alter the resulting amino-acid sequence and consequently affect protein structure or function. However, not every DNA variant changes a protein; some variants are synonymous, occur in non-coding regions, or influence gene regulation instead.

Amino acids are the molecular building blocks of proteins.

For protein-coding genes, the nucleotide sequence ultimately influences the sequence of amino acids incorporated into a protein.

The relationship can be summarized as:

Gene → transcription → mRNA → translation → amino-acid sequence → protein

What Name Is Given to Different Forms of a Gene?

Different forms of a gene are commonly called alleles.

An allele is an alternative sequence form of a gene at a particular genetic locus.

For example, individuals can inherit different alleles from their biological parents. The combination of alleles an individual carries contributes to the individual's genotype, while observable characteristics arising from genotype together with environmental and developmental influences form the phenotype.

This distinction is fundamental to genetics:

Gene: a functional hereditary DNA sequence.
Allele: an alternative form or sequence variant of that gene.

Different forms of a gene are commonly called alleles.

An allele is an alternative sequence form of a gene at a particular genetic locus.

This distinction is fundamental to genetics:

Gene: a functional hereditary DNA sequence.
Allele: an alternative form or sequence variant of that gene.

What Is a Gene Pool?

A gene pool is the total collection of genetic variants, including alleles, present in a population at a particular time.

Gene pools are central to population genetics and evolutionary biology.

Allele frequencies within a gene pool can change over generations through mechanisms including:

mutation
natural selection
genetic drift
migration and gene flow
non-random mating

Studying gene pools helps researchers understand genetic diversity, population structure and evolutionary change.

A gene pool is the total collection of genetic variants, including alleles, present in a population at a particular time.

Gene pools are central to population genetics and evolutionary biology.

Allele frequencies within a gene pool can change over generations through mechanisms including:

mutation
natural selection
genetic drift
migration and gene flow
non-random mating

Studying gene pools helps researchers understand genetic diversity, population structure and evolutionary change.

What Is a Gene? A-Level Biology Explanation

For A-level biology, a useful definition is:

A gene is a sequence of DNA nucleotides that encodes a functional product, such as a polypeptide or functional RNA.

Students should understand the relationships among genes, alleles, chromosomes and proteins.

A gene occupies a particular genomic location, often described as its locus. Alternative versions of a gene are alleles. Protein-coding genes are transcribed into RNA, and their coding information can subsequently be translated into a sequence of amino acids.

It’s also important to understand that phenotype is rarely explained by a simple “one gene = one characteristic” model. Many characteristics are polygenic and are also influenced by environmental factors.

For A-level biology, a useful definition is:

A gene is a sequence of DNA nucleotides that encodes a functional product, such as a polypeptide or functional RNA.

Students should understand the relationships among genes, alleles, chromosomes and proteins.

Where Are Genes Located?

In eukaryotic organisms, most genes are located on chromosomes within the cell nucleus.

Human nuclear DNA is organized into 23 pairs of chromosomes, giving 46 chromosomes in most nucleated somatic cells.

One chromosome of each pair is generally inherited from the biological mother and the other from the biological father.

Humans also possess mitochondrial DNA (mtDNA). Mitochondria contain a small circular genome encoding genes important for mitochondrial function.

In eukaryotic organisms, most genes are located on chromosomes within the cell nucleus.

Human nuclear DNA is organized into 23 pairs of chromosomes, giving 46 chromosomes in most nucleated somatic cells.

One chromosome of each pair is generally inherited from the biological mother and the other from the biological father.

Humans also possess mitochondrial DNA (mtDNA). Mitochondria contain a small circular genome encoding genes important for mitochondrial function.

How Many Genes Do Humans Have?

Humans have roughly 20,000 protein-coding genes, although the exact number depends partly on genome annotation methods and how genes are classified.

The number of genes is much smaller than early estimates made before completion and refinement of the human genome sequence.

Biological complexity therefore does not simply depend on the number of genes. Alternative RNA splicing, gene regulation, non-coding RNAs, protein modification and interactions among genes and regulatory sequences greatly expand biological diversity.

Humans have roughly 20,000 protein-coding genes, although the exact number depends partly on genome annotation methods and how genes are classified.

The number of genes is much smaller than early estimates made before completion and refinement of the human genome sequence.

What Is Gene Expression?

Gene expression is the process through which information encoded by a gene produces a functional product.

Cells do not use every gene at the same level or at the same time.

Different patterns of gene expression help explain why cells containing essentially the same genome can perform very different functions.

For example, a neuron and a muscle cell contain largely the same nuclear DNA, but they express different combinations of genes.

Gene expression can be regulated at numerous stages, including transcription, RNA processing, RNA stability, translation and protein modification.

Gene expression is the process through which information encoded by a gene produces a functional product.

Cells do not use every gene at the same level or at the same time.

Different patterns of gene expression help explain why cells containing essentially the same genome can perform very different functions.

For example, a neuron and a muscle cell contain largely the same nuclear DNA, but they express different combinations of genes.

Gene expression can be regulated at numerous stages, including transcription, RNA processing, RNA stability, translation and protein modification.

What Is a Genetic Mutation?

A mutation is a change in a DNA sequence.

Mutations can involve a single nucleotide or larger regions of DNA. Examples include substitutions, insertions, deletions, duplications and larger structural alterations.

Their biological effects vary considerably. A mutation can be:

neutral, producing little or no detectable functional effect,

harmful, disrupting biological function or contributing to disease,

or, in some contexts, beneficial, increasing reproductive fitness under particular environmental conditions.

Mutations also provide an important source of new genetic variation upon which evolutionary processes can act.

A mutation is a change in a DNA sequence.

Mutations can involve a single nucleotide or larger regions of DNA. Examples include substitutions, insertions, deletions, duplications and larger structural alterations.

Their biological effects vary considerably. A mutation can be:

neutral, producing little or no detectable functional effect,

harmful, disrupting biological function or contributing to disease,

or, in some contexts, beneficial, increasing reproductive fitness under particular environmental conditions.

Mutations also provide an important source of new genetic variation upon which evolutionary processes can act.

Definitions
Questions	Answers
How Can Genes Be Related to Genetic Disease?	Genetic disorders can occur when pathogenic variants alter the function or regulation of genes. For example, cystic fibrosis is associated with pathogenic variants in the CFTR gene. These variants can disrupt the function of the CFTR chloride channel and affect epithelial ion and water transport. However, the relationship between genes and disease is not always simple. Some disorders are primarily associated with variants in a single gene, whereas common conditions can involve many genetic variants together with environmental, developmental and lifestyle factors. Therefore, possessing a particular genetic variant does not always mean that a person will inevitably develop a disease.
Genes and Blood Types	Human blood groups provide a classic example of genetic inheritance. The ABO blood group system is strongly influenced by different alleles of the ABO gene. The major A, B and O allele classes contribute to the familiar A, B, AB and O blood-group phenotypes. The ABO system also illustrates concepts such as multiple alleles and codominance. The A and B alleles are codominant with one another, while many common O alleles produce a nonfunctional or substantially altered glycosyltransferase.
Are All Genes Active in Every Cell?	No. Most cells contain essentially the same nuclear genome, but different cell types activate and repress different sets of genes. This differential gene expression is essential for cellular specialization. A red-blood-cell precursor, neuron, hepatocyte and muscle cell follow different gene-expression programs despite originating from cells containing essentially the same genome. Mature human red blood cells are a notable exception because they lose their nucleus during development and consequently lack nuclear DNA.

Gene vs Allele: What Is the Difference?

Frequently Asked Questions About Genes

What is a gene in simple terms?+

A gene is a section of DNA containing information that contributes to making a functional biological product, usually a protein or functional RNA.

What is a gene made of?+

What is a gene made of?

What name is given to different forms of a gene?+

Different forms of a gene are called alleles.

What is a gene pool?+

A gene pool is the total collection of genetic variants present within a population.

Is a gene the same as DNA?+

No. DNA is the molecule that stores genetic information. A gene is a functional sequence within DNA.

Is a chromosome a gene?+

No. A chromosome is a DNA-protein structure containing many genes as well as regulatory and other DNA sequences.

Do genes make proteins?+

Many genes contain instructions for protein production, but not all genes are protein-coding. Some produce functional RNA molecules.

What determines which genes are active?+

Gene activity is controlled by regulatory DNA sequences, transcription factors, chromatin organization, epigenetic mechanisms, RNA regulation and signals within and outside the cell.

Can genes change?+

DNA sequences can change through mutation. Genetic variation can also arise through recombination and other genomic processes.

How many genes are in the human genome?+

Current genome annotations identify approximately 20,000 protein-coding genes, in addition to numerous genes producing non-coding RNAs.