Genes are sections of DNA that contain information used by cells to make functional products, most often proteins or functional RNA molecules. They are the basic units of heredity: the information in genes can be passed from parents to their children and can influence traits such as blood type, eye color, and susceptibility to certain diseases.
Genes are not simple instructions that independently determine everything about a person. Traits usually arise from interactions among many genes, the environment, and the way cells regulate gene activity. Understanding that distinction is key to understanding what genes actually do.
Where genes are found
In humans, most DNA is packaged into structures called chromosomes inside the nucleus of a cell. Each chromosome is a long DNA molecule associated with proteins that help organize and package it.
Humans typically have 23 pairs of chromosomes, for a total of 46 in most cells. One chromosome in each pair is inherited from the mother and the other from the father.
Genes occupy particular locations along chromosomes. A gene can be thought of as a defined stretch of DNA, although the DNA surrounding and regulating a gene also plays important roles in controlling when and where that gene is active.
There is also a small amount of DNA in mitochondria, structures that produce much of a cell’s usable energy. Mitochondrial DNA contains genes that are involved in mitochondrial function and is generally inherited through the mother.
What DNA has to do with genes
DNA, short for deoxyribonucleic acid, is the molecule that stores genetic information. It is built from four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G).
The order of these bases forms biological information. Because the bases pair in specific ways—A with T and C with G—DNA can be copied when cells divide.
A gene is therefore not a separate substance from DNA. A gene is a particular sequence of DNA that carries information the cell can use.
Some genes provide the instructions for making proteins. Others produce RNA molecules that have functions of their own. In addition, genes are controlled by regulatory DNA sequences that help determine when a gene is turned on, where it is active, and how much of its product is made.
How genes work
For a protein-coding gene, the basic process begins when a cell uses the DNA sequence as a template to make a related molecule called messenger RNA (mRNA). This step is called transcription.
The mRNA can then be used by a cellular structure called a ribosome to assemble a protein from amino acids. This step is called translation.
The protein may then serve as an enzyme, structural component, signaling molecule, receptor, or another type of cellular machinery. Its activity can affect how cells function and, ultimately, how an organism develops and operates.
This flow of information is often summarized as:
DNA → RNA → protein
That summary is useful, but it is not the whole story. Not every gene produces a protein, and cells have complex systems that regulate gene activity. The same DNA can therefore be used differently in different types of cells.
What is an allele?
A gene can exist in slightly different DNA sequences called alleles.
For example, a gene involved in a biological trait may have several common versions in a population. A person can inherit one allele from each parent for genes located on paired chromosomes.
Some alleles have little or no noticeable effect. Others can alter how a gene works and therefore influence a trait or contribute to a genetic condition.
The relationship between alleles and traits can be more complicated than simply having a “dominant” or “recessive” version. Some traits involve multiple genes, and some genes have effects that depend on the person’s other genes or environment.
How genes are inherited
When humans reproduce, specialized cells called egg and sperm cells carry one set of chromosomes rather than the two sets found in most body cells. When an egg and sperm combine, the resulting cell receives genetic material from both parents.
As a result, a child receives roughly half of their nuclear chromosomes from each biological parent. The particular combination of genetic variants inherited from each parent contributes to the child’s genetic makeup.
Inheritance is not always as straightforward as receiving one version of every gene from each parent. Chromosomes exchange segments during the formation of egg and sperm cells, creating new combinations of genetic variants. In addition, new DNA changes can sometimes arise rather than being inherited from a parent.
Genes and traits
A trait is a characteristic of an organism. Some traits are strongly influenced by a single gene, but many human traits are influenced by numerous genes.
Height is one example of a complex trait. Many genetic variants can contribute to differences in height, while nutrition, health, and other environmental factors also matter.
Even traits that have a substantial genetic component are not necessarily controlled by a single gene. Saying that a trait is “genetic” generally means that differences in genetic makeup contribute to differences in that trait; it does not mean that the trait is fixed or determined entirely by DNA.
The same principle applies to many diseases. Some conditions result primarily from changes in a single gene, while others arise from combinations of many genetic variants and environmental influences.
What mutations are
A mutation is a change in DNA. Scientists often use the broader term genetic variant for a difference in DNA sequence, because not every DNA change is harmful or even noticeable.
A genetic change can have several possible effects. It may have no meaningful effect, alter how a gene functions, or in some cases contribute to disease. Some changes can also be beneficial in a particular environment.
DNA changes can occur when DNA is copied or repaired. They can also result from exposure to certain environmental factors. Changes that occur in cells that give rise to eggs or sperm can potentially be passed to future generations. Changes that occur in other body cells generally affect only that individual and are not inherited by their children.
Do genes determine who you are?
Genes are important, but they do not act as a complete blueprint that independently determines every aspect of a person.
Genes influence how cells develop and function, but gene activity is affected by many factors. Environmental conditions, nutrition, physical activity, exposure to certain substances, infections, stress, and other biological processes can all influence outcomes depending on the trait.
Genes can also influence how a person responds to their environment. In this sense, heredity and environment are not competing explanations. They often work together.
A person’s genetic makeup can influence characteristics and risks without making an outcome inevitable. This is especially important when thinking about common diseases and complex traits.
How genes are turned on and off
Cells in the human body generally contain the same genome, yet a nerve cell behaves very differently from a muscle cell or a liver cell. One major reason is that different cells use different sets of genes.
Gene expression refers to the process by which information in a gene is used to produce a functional product. Cells can regulate gene expression at several stages, controlling which genes are active and how strongly they are expressed.
Chemical modifications to DNA and associated proteins can also influence gene activity without changing the underlying DNA sequence. These regulatory processes are part of epigenetics.
Gene regulation allows cells with the same genetic material to develop specialized structures and perform different jobs.
Why genes matter in health and medicine
Genes can help explain why some inherited conditions run in families and why people can respond differently to certain diseases or treatments.
Some genetic disorders are caused by changes in a single gene. Others involve abnormalities affecting chromosomes or combinations of many genetic variants. Genetic differences can also influence susceptibility to common conditions such as heart disease, diabetes, and some cancers, although these conditions usually involve multiple genetic and environmental factors.
Modern medicine can sometimes use genetic information to help diagnose inherited conditions, estimate disease risk, guide certain treatments, or identify genetic changes in tumors. Genetic information is powerful, but interpreting it often requires context: a DNA variant does not automatically translate into a specific outcome.
Genes are information, not destiny
The central idea is simple: genes are DNA sequences that carry biological information and can be inherited from one generation to the next.
Genes help cells make proteins and functional RNA molecules, regulate biological processes, and contribute to the traits and health characteristics of living organisms. Different versions of genes can produce genetic differences among people, while gene regulation and environmental factors shape how those differences are expressed.
Understanding genes therefore means looking beyond the idea of a single “gene for” a particular characteristic. Heredity is a dynamic system in which DNA, gene regulation, cells, development, and the environment interact to produce the biology of an individual.
