Humans are remarkably similar genetically, yet no two unrelated people are genetically identical. Those differences help explain why people can have different blood types, physical traits, disease risks, responses to medications, and other biological characteristics.
The basic reason is simple: human populations carry many different versions of genes, and each person inherits a particular combination of those variants from their parents. But the origins of genetic variation are more interesting than that explanation suggests. Mutations create new genetic variants, reproduction reshuffles existing ones, populations change over generations, and natural selection can favor some variants in particular environments.
Genetic differences are also more limited than everyday observations might imply. Humans share the overwhelming majority of their DNA, and traits such as height, skin color, and susceptibility to many diseases usually result from interactions among many genes and the environment rather than from a single genetic difference.
Most human DNA is shared
Every human genome contains roughly three billion DNA base pairs—the chemical “letters” that make up DNA. Most of those sequences are the same from person to person because humans share a common evolutionary history and have the same basic biological requirements.
Still, the genome contains many locations where DNA can differ among individuals. A common type of difference is a single-nucleotide variant, in which one DNA letter differs between people. Other differences include insertions or deletions of DNA, repeated segments, and larger structural changes in which pieces of chromosomes are rearranged, duplicated, or missing.
Some genetic differences have little or no noticeable effect. Others influence biological traits by changing how a gene works, how much of a protein is produced, or how cells regulate their activity.
This means that being genetically different does not imply being fundamentally different as a species. Individual variation exists within a shared human genetic framework.
Mutations create new genetic variation
The ultimate source of new genetic variants is mutation: a change in DNA.
Mutations can arise when DNA is copied during cell division, when DNA is damaged and repaired imperfectly, or through other biological processes. Mutations also occur in reproductive cells, such as sperm or eggs, and can therefore be passed to offspring. A mutation that occurs in an ordinary body cell generally affects only that individual and is not inherited by their children.
Most mutations have little effect on a person’s observable characteristics. Some are harmful, some can be beneficial under particular circumstances, and many are effectively neutral. A mutation’s consequences depend on where it occurs, what it changes, and the biological and environmental context.
Mutation therefore supplies the raw material for genetic diversity, but mutation alone does not explain why people inherit such different combinations of variants.
Sexual reproduction reshuffles existing variants
When a child is conceived, they receive roughly half of their nuclear DNA from each biological parent. But they do not receive an intact half of one parent’s genome and an intact half of the other’s.
During the production of eggs and sperm, chromosomes undergo recombination. Matching chromosome pairs exchange segments of DNA, creating chromosomes with new combinations of genetic variants. The particular egg and sperm that unite then contribute another layer of randomness.
As a result, siblings can inherit substantially different combinations of their parents’ genetic variants. Identical twins are a special case: they originate from the same fertilized egg and therefore begin with nearly the same DNA, although biological processes can produce some differences between them over time.
Recombination is especially important because it continuously rearranges genetic variation that already exists in a population.
Inheritance is why family members resemble one another—and differ
Genes are inherited through chromosomes, and many genes occur in different versions called alleles. A person can inherit different alleles of the same gene from their two biological parents.
For some characteristics, those differences have relatively straightforward effects. The ABO blood group system, for example, depends on inherited variants that help determine whether a person’s red blood cells carry particular molecular markers.
Most traits, however, are more complicated. Characteristics such as height, blood pressure, and many aspects of disease susceptibility are polygenic, meaning they are influenced by variants at many different locations in the genome. Environmental factors can matter just as importantly or even more importantly for particular traits.
This is why it is usually misleading to look for a single “gene for” a complex characteristic. Genes generally contribute to biological tendencies and processes rather than functioning as simple switches for most human traits.
Populations accumulate different genetic variants
Human genetic diversity also reflects the history of populations.
For much of human history, people lived in populations that were partly separated by geography. Individuals within a population tended to have more opportunities to reproduce with one another than with people living far away. Over many generations, different populations could therefore develop different frequencies of genetic variants.
Several evolutionary processes contribute to these differences.
Genetic drift occurs when the frequencies of variants change partly because of chance. Its effects can be particularly strong in small populations. A variant can become more or less common simply because some individuals happen to leave more descendants than others.
Natural selection can also change variant frequencies. If a genetic variant affects survival or reproduction in a particular environment, individuals carrying it may, on average, leave more descendants. A variant that is advantageous in one environment is not necessarily advantageous everywhere.
Gene flow works in the opposite direction by moving genetic variants between populations when people migrate and have children with members of other populations. Throughout human history, migration and intermarriage have repeatedly mixed previously separated populations.
These processes operate together. Human genetic variation is therefore a record of mutation, reproduction, migration, population history, chance, and selection acting over many generations.
Human genetic variation does not divide neatly into races
Human populations do differ in the frequencies of some genetic variants, particularly when populations have had different geographic histories and environmental pressures. But genetic variation does not fall into a set of clean biological boxes corresponding to commonly used racial categories.
Most human genetic variation exists within populations rather than exclusively between them. Populations also overlap genetically, and human migration and intermarriage have occurred throughout our history.
Traits that vary geographically can sometimes give the impression of sharp biological boundaries. Skin pigmentation is a useful example. Its geographic distribution reflects inherited genetic variation shaped partly by adaptation to differences in ultraviolet radiation. Yet pigmentation itself varies continuously across populations, and the genes involved are not confined to particular racial groups.
Race can be socially and historically important, but it is not a precise substitute for an individual’s genetic ancestry. Genetic ancestry instead describes the populations from which a person’s DNA was inherited and can be more informative when a genetic question specifically concerns population history or inherited disease variants.
Environment also helps produce differences between people
Genetic differences are only part of the explanation for variation in human characteristics.
A person’s phenotype—the observable result of their biology—can reflect both inherited genetic variation and environmental influences. Nutrition, infections, physical activity, exposure to sunlight, medications, socioeconomic conditions, stress, and many other factors can affect biological outcomes.
Genes and environment also interact. A genetic variant may have different effects depending on environmental conditions. For example, the amount of a particular nutrient or exposure to a particular substance can influence whether a genetic difference has noticeable consequences.
This is why genetic influence should not be confused with genetic destiny. A trait can be substantially influenced by genetics without being fixed or inevitable.
Why do some genetic differences persist?
If a genetic variant is harmful, it might seem that natural selection should eventually eliminate it. Sometimes that happens, but evolution is not an optimization process that removes every disadvantageous variant.
Some harmful variants persist because they have only a small effect on reproduction, arise repeatedly through mutation, or are recessive and therefore can remain hidden in people who carry one copy. A variant can also be harmful in one circumstance while providing an advantage in another.
Chance matters as well. Genetic drift can preserve or eliminate variants independently of whether they are beneficial or harmful, particularly in small populations.
Evolution therefore produces populations containing a mixture of variants with different effects—not a perfectly optimized set of genes.
Genetic differences can affect health, but risk is not the same as certainty
Genetic variation is one reason people differ in their susceptibility to diseases.
Some conditions are strongly influenced by variants in a single gene. In those cases, inheritance can have a relatively direct relationship with disease risk, although even single-gene disorders can show variation in severity or whether symptoms appear.
Many common conditions are different. Heart disease, diabetes, cancer, and many psychiatric and neurological conditions typically involve numerous genetic variants, environmental exposures, and biological processes. Each individual variant may have a small effect, while their combined influence interacts with lifestyle and other circumstances.
A genetic variant associated with greater risk therefore does not necessarily mean that a person will develop the condition. Conversely, having no known risk variant does not guarantee protection.
Genetic testing can identify particular inherited variants, but interpreting what they mean requires attention to the specific gene, variant, inheritance pattern, ancestry, environment, and available medical evidence.
Genetic differences are part of what evolution needs to work
Variation is essential to evolution by natural selection. If every individual had exactly the same DNA, there would be little inherited variation for selection to act on.
Mutations introduce new variants. Recombination and sexual reproduction generate new combinations of those variants. Migration moves variants among populations. Natural selection and genetic drift then alter their frequencies over generations.
The result is a continually changing distribution of genetic variation.
Human genetic diversity is therefore not a collection of unrelated differences that appeared independently in modern people. It is the accumulated product of billions of years of evolution, followed by the more recent evolutionary and demographic history of our own species.
The striking fact is not simply that humans are genetically different from one another. It is that we combine substantial genetic diversity with an extraordinary degree of genetic similarity. That combination allows individuals to differ in meaningful biological ways while remaining members of one closely related species.