Organisms get their traits from the DNA passed down by their parents, combined with the environmental conditions they experience during development and throughout life. Every living thing carries a set of genetic instructions encoded in its DNA, but those instructions don’t operate in isolation. The traits you actually see in an organism, from eye color to height to behavior, emerge from an ongoing interaction between genes and environment.
DNA, Genes, and Chromosomes
Inside nearly every cell of every organism sits a molecule called DNA. DNA is structured as two complementary strands twisted together, and each strand contains all the information needed to reconstruct the other. Stretches of DNA that code for specific proteins are called genes, and genes are bundled together on larger structures called chromosomes. Each gene specifies the structure of a single protein (or part of one), and those proteins do the actual work of building and running the body. The color of a flower petal, the shape of a beak, the texture of your hair: all of these start as instructions written in DNA.
Organisms typically carry two copies of each gene, one inherited from each parent. These copies don’t have to be identical. Different versions of the same gene are called alleles, and which combination of alleles you inherit determines the genetic starting point for a given trait.
How Traits Pass From Parent to Offspring
The basic rules of inheritance were worked out by Gregor Mendel in the 1800s using pea plants, and they still hold up remarkably well. Mendel identified three core principles that explain how traits move between generations.
- Dominance: Some alleles are dominant, meaning their effect shows up whenever they’re present. Others are recessive, meaning they only produce a visible trait when an organism carries two copies. This is why a trait can seem to skip a generation: both parents can silently carry a recessive allele and pass it to a child who inherits two copies.
- Segregation: When an organism produces eggs or sperm, its two copies of each gene split apart so that each reproductive cell carries only one copy. This happens randomly, giving each allele a 50/50 chance of being passed on.
- Independent assortment: Genes for different traits are sorted into reproductive cells independently of one another. The version of the gene you pass on for seed shape, to use Mendel’s example, has no influence on which version you pass on for seed color.
These principles explain straightforward, single-gene traits well. But most of the traits we notice in everyday life are more complicated.
Most Traits Involve Many Genes
Traits like height, weight, skin color, and intelligence don’t follow simple dominant-or-recessive patterns. They’re polygenic, meaning dozens, hundreds, or even thousands of genes each contribute a small effect. The mathematician R. A. Fisher showed in 1918 that you could explain the smooth, continuous variation seen in traits like height by assuming a large number of genes, each inherited according to Mendel’s rules but each having only a tiny individual impact.
Modern genetic studies confirm this pattern. When researchers scan the genome for variants associated with complex traits, they overwhelmingly find many variants of small effect rather than a few genes with large effects. This is why traits like height or personality don’t sort neatly into categories the way Mendel’s round and wrinkled peas did. Instead, they fall on a spectrum, with most individuals clustering somewhere in the middle.
Environment Shapes How Genes Are Expressed
Genes provide instructions, but the environment determines how those instructions play out. Whether a particular gene is active, and how strongly it’s active, depends heavily on the conditions the organism experiences. Nutrition, stress, temperature, sunlight, social interactions: all of these can dial gene activity up or down.
Some of the most striking evidence comes from prenatal development. Conditions experienced by a mother, including protein deficiency, excessive stress, serious infection, smoking, and alcohol use, can alter gene activity in the developing fetus. Specifically, high levels of stress hormones in the mother can suppress genes involved in growth, contributing to low birth weight and increased sensitivity to stress later in the offspring’s life. Studies in rats found that the amount of physical contact and nursing a mother provided directly shaped how her pups’ stress-response genes functioned into adulthood. Pups that received more attentive care grew up with lower stress hormone levels when challenged.
In monkeys, researchers tested the effect of unpredictable food availability on mother-infant pairs. When foraging conditions shifted randomly between easy and difficult, the disruption to maternal behavior was far greater than in either consistently easy or consistently hard conditions. The unpredictability itself was the damaging factor, altering how mothers interacted with their young and, through that, how the infants developed.
The Same Genes Can Produce Different Traits
One of the most powerful demonstrations that genes alone don’t dictate traits comes from organisms with identical DNA producing visibly different outcomes in different environments. Plants are especially good at this. A single plant genotype can change its sex or shift how much energy it puts into male versus female flowers in response to environmental stress. When soil is rich in certain nutrients like nitrate, plants activate genes that promote root branching toward the nutrient source, physically reshaping their root system. When pollinators are scarce, some plants maintain larger floral displays to attract them, then wilt flowers after pollination to prevent self-pollination when pollinators are abundant.
Animals show this flexibility too. Genetically identical colonies of hydroids, small marine animals, grow into completely different physical forms depending on the chemical environment of their cells. Shifting the internal chemistry one direction produces long, runner-like growth. Shifting it the other direction produces flat, sheet-like growth. Same DNA, dramatically different body.
Epigenetics: Changes Above the DNA
Between the gene and the trait sits a layer of chemical controls that can turn genes on or off without changing the DNA sequence itself. These are called epigenetic mechanisms, and three main types have been identified.
The first is DNA methylation. Enzymes attach a small chemical tag (a methyl group) directly to certain DNA bases. When these tags accumulate in the regulatory region of a gene, they effectively silence it, preventing the cell’s machinery from reading that gene’s instructions. The second mechanism involves modifications to histone proteins, the spool-like structures that DNA wraps around. Adding or removing chemical groups to histones changes how tightly the DNA is wound. Loosely wound DNA is accessible and can be read; tightly wound DNA is shut off. Different modifications, including the addition of acetyl, methyl, or phosphate groups, push the packaging in one direction or the other. The third mechanism involves small molecules of non-coding RNA that help guide both DNA methylation and histone modification.
Epigenetic changes are one reason why identical twins, who share the same DNA, can develop different traits over time. Their experiences gradually create different patterns of gene activation across their cells.
How Much Is Genes, How Much Is Environment?
The relative contribution of genetics and environment varies by trait. For the “Big Five” personality traits (openness, conscientiousness, extraversion, agreeableness, neuroticism), twin studies estimate that genetics accounts for 40 to 60 percent of the variation between people. But when researchers look at which specific DNA variants are responsible, they can only identify a fraction of that. For neuroticism, common genetic variants explain roughly 15 percent of the variation; for openness, about 21 percent. The gap between these numbers and the twin-study estimates suggests that many genetic influences are still too small or too complex to pinpoint individually.
Height is one of the most heritable human traits, with genetics explaining roughly 80 percent of the variation in well-nourished populations. But even height is sensitive to environment: average heights have increased dramatically in populations that shifted from poor to adequate nutrition over a few generations, far too fast for genetic change to explain.
Mutations Create New Traits
All genetic variation ultimately traces back to mutations, random changes in DNA that occur when cells copy their genetic material. These changes include single-letter swaps in the DNA code, insertions of extra letters, or deletions of existing ones. Most mutations have no noticeable effect, and many are harmful. But occasionally, a mutation produces a protein that works slightly differently in a way that benefits the organism, and natural selection can spread that variant through a population over generations.
While mutations are the raw source of all new genetic diversity, their impact on trait variation in any single generation is small. The bulk of the trait differences you see between individuals in a population come from alleles that have been circulating for many generations, shuffled into new combinations through sexual reproduction.
Traits Acquired Outside Normal Inheritance
Not all organisms get their traits exclusively from their parents. Bacteria regularly pick up genes from unrelated organisms through a process called horizontal gene transfer. This happens in three ways: bacteria absorb loose DNA fragments from their surroundings (transformation), receive genes directly from another bacterial cell through a physical bridge (conjugation), or have genes delivered by viruses that infect bacteria (transduction). Horizontal gene transfer is the primary way antibiotic resistance spreads among bacterial populations, which is why a species of bacteria can go from fully treatable to drug-resistant in a matter of years.
Human Selection as a Source of Traits
Humans have been deliberately reshaping the traits of other organisms for thousands of years through selective breeding. The process is straightforward: choose the individuals with the traits you want and breed them, repeating over many generations. The results can be dramatic. Cauliflower, broccoli, cabbage, kale, and kohlrabi all descend from a single species of wild mustard. Each crop was selectively bred to exaggerate a different part of the plant: the flower clusters, the leaves, the stem.
Dogs offer an even more striking example. Domesticated from wolves roughly 20,000 to 40,000 years ago, domestic dogs have been shaped into breeds so physically different from one another that, without knowing their history, we might classify them as entirely separate species. Many of these breeds were developed in just the last 200 years. Dachshunds were selected to hunt burrowing animals, German Shepherds to herd sheep. The underlying mechanism is the same as natural selection: certain genetic variants are favored over others. The difference is that humans, rather than environmental pressures, decide which variants get passed on.