Traits don’t come from genes alone or from the environment alone. They come from both acting together. Researchers now treat nature versus nurture as a partnership: genes limit what the environment can do, and the environment changes how genes behave.1PubMed Central. The Nature-Nurture Debate and Public Policy Genes set the range of possible outcomes, and food, temperature, stress, chemicals and even gut microbes help decide where in that range an organism lands. Some differences come from neither, only from chance during development.
Genes Set the Range and the Environment Picks the Outcome
A gene version works less like a fixed blueprint and more like a recipe that adapts to conditions. For many traits, one set of genes produces different results in different environments, and the pattern can be smooth and gradual or an either-or switch.2Peer Community Journal. Partitioning the phenotypic and genetic variances of reaction norms Different genetic types can also respond to the same change in conditions in different ways.3Peer Community Journal. Partitioning the phenotypic and genetic variances of reaction norms Some genes make an organism highly sensitive to its surroundings, and others make it steady.
One way to picture this is a graph with the environment (say, nutrition level) along the bottom and the trait (say, adult height) up the side. Each genetic type traces its own line across it. Flat lines describe a steady genotype, steep lines a sensitive one, and lines that cross show that the “best” genotype can change with conditions. That’s why you can’t rank two genotypes as better or worse without saying under which conditions, because the ranking can flip.
Asking what percent is genetic is like asking whether a rectangle’s area comes more from its length or its width. The answer can even differ between populations because their environments differ. What can be asked is how much the differences between individuals track genes versus surroundings.
When the same experience affects people differently depending on their genes, it is called gene-environment interaction. Across 20 groups of boys and men, childhood maltreatment was more strongly linked to later antisocial behavior in those with a low-activity version of a gene called MAOA.4PubMed Central. MAOA, childhood maltreatment and antisocial behavior: Meta-analysis of a gene-environment interaction In boys, physical abuse had a much weaker effect on mental health among those with the high-activity version.5Molecular Psychiatry. MAOA, maltreatment, and gene–environment interaction predicting children’s mental health: new evidence and a meta-analysis In women and girls, the pattern was weak or absent.6PubMed Central. MAOA, childhood maltreatment and antisocial behavior: Meta-analysis of a gene-environment interaction These findings describe average patterns across groups, not a prediction for any one person. Having the low-activity version does not make someone antisocial.
Striking Examples of Environment Changing an Animal’s Body
One set of genes can produce different forms depending on surroundings. Predators are a strong trigger. Water fleas grow small spiky “neckteeth” that defend them when they sense the larvae of predatory midges.7PubMed Central. Predator‐induced shape plasticity in Daphnia pulex Tadpoles quickly slow down when they smell predators, and with lasting exposure they grow smaller bodies and larger tails.8Frontiers in Endocrinology. STRESS HORMONES MEDIATE PREDATOR-INDUCED PHENOTYPIC PLASTICITY IN AMPHIBIAN TADPOLES A bigger tail may draw strikes away from the body or help them dart away.9Frontiers in Endocrinology. STRESS HORMONES MEDIATE PREDATOR-INDUCED PHENOTYPIC PLASTICITY IN AMPHIBIAN TADPOLES
Temperature can decide sex and color. In many turtles and alligators, the temperature of the nest decides whether an egg hatches male or female. A temperature-sensing gene in turtles appears to translate warmth into a signal to build testes or ovaries.10Genes to Genomes. First gene linked to temperature-dependent sex determination In red-eared slider turtles, blocking one gene-switching protein at a male-producing temperature turned most embryos female.11Wikipedia. Temperature-dependent sex determination In alligators, a gene’s message is processed differently at female-producing temperatures within 3 days of a temperature shift.12PubMed Central. Differential intron retention in Jumonji chromatin modifier genes is implicated in reptile temperature-dependent sex determination
The temperature acts during a sensitive window in the middle of incubation, not at one instant, so brief warm or cool spells don’t necessarily flip the outcome. Humans and most mammals work differently: chromosomes inherited at fertilization set sex, so the reptile effect is a separate system, not a variant of ours. Because nest temperature is shaped by shade, depth and season, a warming climate can skew the ratio of males to females in a population.
Some butterflies emerge with different wing colors depending on the season, driven by temperature and day length together.13bioRxiv. Seasonal polyphenism of wing colors and its influence on sulphur butterfly diversification Darker spring wings help cool-season butterflies absorb heat, and siblings raised in warm long days versus cool short days developed different patterns.14The American Naturalist. Seasonal Polyphenism in Wing-Melanin Pattern and Thermoregulatory Adaptation in Pieris Butterflies
Food and soil reshape bees and plants. Honeybee larvae are genetically alike, yet those fed royal jelly become queens while the rest become workers.15PubMed Central. Histone deacetylase inhibitor activity in royal jelly might facilitate caste switching in bees A fatty acid making up to 5% of royal jelly acts on the chemical switches that control genes.16PubMed Central. Histone deacetylase inhibitor activity in royal jelly might facilitate caste switching in bees Plant roots change their branching and length depending on which nutrient is scarce.17PubMed Central. Plasticity of the Arabidopsis Root System under Nutrient Deficiencies Nutrient levels vary across soil and time, so roots rebuild themselves to find what they need.18PubMed. Nutrient-hormone relations: Driving root plasticity in plants A houseplant grown in dim light turns tall and spindly, while the same plant in sun stays compact. Hydrangea flowers shift between blue and pink with soil acidity.
Changing shape pays off as a survival strategy. Growing defenses costs energy, so making them only when predators are present saves resources, and an organism that adjusts can live in more varied places. The downside is a lag, since the change takes time to develop. If the cue misleads, the organism builds the wrong body for its conditions.
How the Environment Switches Genes On and Off
Epigenetics refers to chemical tags on DNA and its packaging that turn genes on or off without changing the DNA code. A tag on or near a gene can make it easier or harder for the cell to read that gene. DNA is wrapped around proteins, and tags on those proteins loosen or tighten the wrapping, which changes how reachable a gene is. Every cell in your body holds the same DNA, and different patterns of tags are a major reason a skin cell and a nerve cell behave differently.
In honeybees, these DNA tags differ between queens and workers, linked to which genes each caste uses more.19PubMed Central. DNA methylation is widespread and associated with differential gene expression in castes of the honeybee, Apis mellifera Plants and trees widely respond to drought, heat, and other stress by changing these tags, which helps them adjust to changing conditions.20PubMed Central. Epigenetic responses of trees to environmental stress in the context of climate change In threatened plants with little genetic variety, the tag changes matched how strongly the plants changed in response to stress.21PubMed. How do threatened plant species with low genetic diversity respond to environmental stress? Insights from comparative conservation epigenomics and phenotypic plasticity
Some environmentally caused changes can be passed on. In asexual dandelions, stress such as attack by pests triggered many tag changes, and many were passed to offspring.22PubMed. Stress-induced DNA methylation changes and their heritability in asexual dandelions In lab animals, chemical exposure can alter tags in sperm and cause disease in later generations that were never exposed.23PubMed Central. Environmentally Induced Epigenetic Transgenerational Inheritance of Reproductive Disease24PubMed Central. Environmental induced transgenerational inheritance impacts systems epigenetics in disease etiology These inherited effects have been shown to affect fertility and reproductive health.25PubMed Central. Environmentally Induced Epigenetic Transgenerational Inheritance of Reproductive Disease DNA tags, changes to DNA packaging, and small RNA molecules are the leading candidates for how a father’s environment reaches his offspring.26Reproductive and Developmental Medicine. Environmentally Induced Paternal Epigenetic Inheritance and Its Effects on Offspring Health Much of this evidence comes from plants and lab animals, though, and how strongly it applies to people is still unsettled.
Many tags reset between generations, so inheritance is the exception, not the rule. Popular claims that you can easily rewrite your genes through lifestyle go beyond the evidence. What is solid is that experience changes which genes are active.
Hormone-disrupting chemicals are an environmental factor that affects development. Some chemicals in the environment interfere with hormones and raise the risk of cancer, reproductive problems, learning difficulties, and obesity.27PubMed Central. Consensus on the key characteristics of endocrine-disrupting chemicals as a basis for hazard identification Some bind to the same cell receptors that steroid hormones use and can disturb how organs form in a developing fetus.28PubMed Central. Developmental effects of endocrine-disrupting chemicals in wildlife and humans Such chemicals are found in consumer goods, personal care products, food, and drinking water, though human studies on children remain limited.29PubMed Central. Exposure to Environmental Endocrine Disruptors and Child Development
To cut exposure, avoid heating food in plastic containers in the microwave, choose fragrance-free personal care products when possible, and wash your hands before eating, especially after handling receipts or chemicals. Pregnancy is the period when limiting exposure matters most.
How Much of Human Traits Comes From Genes
Heritability is how much of the difference between people in a trait is linked to differences in their genes. To estimate it, researchers compare identical and fraternal twins, including twins raised apart. That lets them estimate the shares from genes, shared home life, and individual experience.30PubMed Central. Twin Studies: A Unique Epidemiological Tool
The logic is straightforward. Identical twins share nearly all their varying DNA while fraternal twins share about half, so if identical pairs are more alike in a trait, genes are likely playing a part. If both kinds of twins are about equally alike, shared home life looks more important. Twins raised apart help because they share genes but not a home, so their similarities are harder to credit to upbringing.
Most behavioral traits turn out to be partly heritable.31PubMed. Genetic and environmental influences on human behavioral differences Yet the environmental factors that matter most are usually the ones siblings do not share, such as different friends or experiences.32PubMed. Genetic and environmental influences on human behavioral differences Each child has a different place in the family and different treatment. Teachers, illnesses and chance events differ too, and siblings share only about half their varying genes unless they are identical.
The genetic share differs greatly by trait. Openness and conscientiousness showed substantial genetic influence and little effect from the shared family home.33PubMed. Genetic and environmental effects on openness to experience, agreeableness, and conscientiousness: an adoption/twin study For agreeableness, genes accounted for only 12% of differences and shared upbringing for 21%.34PubMed. Genetic and environmental effects on openness to experience, agreeableness, and conscientiousness: an adoption/twin study For body weight measured as BMI, gene activity in belly fat accounted for about two-thirds of differences between people.35PubMed. Phenotypic variance partitioning by transcriptomic gene expression levels and environmental variables for anthropometric traits using GTEx data The same calculation using blood instead of fat tissue explained only about 16%.36PubMed. Phenotypic variance partitioning by transcriptomic gene expression levels and environmental variables for anthropometric traits using GTEx data People with a lower genetic tendency toward high BMI appeared more affected by factors like smoking and drinking.37PubMed. Phenotypic variance partitioning by transcriptomic gene expression levels and environmental variables for anthropometric traits using GTEx data
Counting interactions shrinks the genetic share. For schizophrenia, a simple model put the genetic share at 79%.38PubMed Central. Dissecting schizophrenia phenotypic variation: the contribution of genetic variation, environmental exposures, and gene–environment interactions A model that included environment and gene-environment interaction lowered it to 46%, with interactions explaining about 35%.39PubMed Central. Dissecting schizophrenia phenotypic variation: the contribution of genetic variation, environmental exposures, and gene–environment interactions A heritability figure depends on how it is calculated and is not a fixed property of the trait.
Heritable does not mean unchangeable: height is highly heritable yet rose with better nutrition. A heritability figure describes a population, not how much of your own trait came from genes, and the same trait can have different heritability in different environments. Low heritability doesn’t mean genes are unimportant to the trait’s biology either.
Why DNA Tests Predict Traits Poorly
Genetic risk scores add up the tiny effects of many DNA variants.40PubMed Central. Missing heritability, polygenic scores, and gene–environment correlation Yet the score for BMI explains less than 2% of differences in BMI, far below its heritability.41PubMed Central. Missing heritability, polygenic scores, and gene–environment correlation Individual variants have very small effects, and rarer variants and gene-environment interactions are largely missed.42PubMed Central. IPRS: Leveraging Gene-Environment Interaction to Reconstruct Polygenic Risk Score43PubMed Central. Missing heritability, polygenic scores, and gene–environment correlation Part of the shortfall comes from studies not yet being large enough to capture many small effects.44PubMed Central. Predictive accuracy of combined genetic and environmental risk scores Adding environmental and lifestyle information to genetic scores can improve predictions.45PubMed Central. IPRS: Leveraging Gene-Environment Interaction to Reconstruct Polygenic Risk Score46PubMed Central. Predictive accuracy of combined genetic and environmental risk scores
Heritability comes from comparing relatives and covers all genetic influence, while a score only adds up the variants researchers have identified and measured. So the gap is a measure of what current tools do not capture, not proof that genes are unimportant. Scores should improve as studies grow, but they are unlikely to become a crystal ball for traits that depend heavily on environment.
That shapes how to read a consumer DNA trait report. A result like “likely to have higher BMI” shifts odds slightly and does not fix your outcome, and lifestyle and surroundings usually predict weight better than a genetic score. Single-gene conditions are the exception, where DNA tests are often strongly predictive. Health-related results are worth discussing with a doctor or genetic counselor before you act on them.
When Chance Shapes a Trait
Even when genes and environment are identical, random events in cells and molecules during development add variation.47PubMed Central. Isolating and quantifying the role of developmental noise in generating phenotypic variation Some of this randomness is true chance in molecules, and some comes from complex feedback within growing tissues.48Symmetry. Nature, Nurture, and Noise: Developmental Instability, Fluctuating Asymmetry, and the Causes of Phenotypic Variation The effect of chance can itself depend on genes and temperature, so the three sources interact.49Frontiers in Ecology and Evolution. Phenotypic Plasticity, Developmental Instability, and Robustness: The Concepts and How They Are Connected Models that split traits into genes and environment therefore always leave a leftover share neither predicts.50Peer Community Journal. Partitioning the phenotypic and genetic variances of reaction norms
Cells contain small numbers of certain molecules, so whether a gene is switched on at a given moment can come down to chance encounters. Tiny early differences can be magnified as tissues grow, because each stage builds on the one before it. That’s why genetically identical organisms raised side by side in carefully matched conditions still turn out slightly different.
Identical twins have different fingerprints despite sharing genes and a womb. The left and right sides of a face or body are never perfectly symmetrical, and cloned cats can have different coat patterns from their donor.
What This Means for Species in a Changing Climate
Over generations, a trait first triggered by the environment can become built in and appear even without the trigger.51PubMed Central. A theoretical perspective on Waddington’s genetic assimilation experiments If a trait is triggered in every generation and is useful, selection may favor individuals who produce it more readily. The threshold for triggering can then drop until the trait appears without the original cue. Flexibility can be a stepping stone to a fixed adaptation, not only a stand-in for it.
Populations adapted to different climates often differ in how they respond to the environment.52PubMed Central. Adaptation to climate change through genetic accommodation and assimilation of plastic phenotypes Genetic variety in these responses may therefore predict which species are vulnerable to climate change.53PubMed Central. Adaptation to climate change through genetic accommodation and assimilation of plastic phenotypes In one bird population, strong flexibility left little genetic variety in how birds respond, limiting their ability to evolve.54PubMed Central. Strong phenotypic plasticity limits potential for evolutionary responses to climate change That matters because more erratic weather makes seasonal cues like temperature less reliable.55PubMed Central. Strong phenotypic plasticity limits potential for evolutionary responses to climate change
A reliable cue can turn into a trap. Many animals time breeding by temperature or day length as a proxy for food. If spring warms earlier but food appears at a different time, timing becomes mismatched. Day length does not change with climate, so species relying on it may fall out of step. Species with varied responses have a better chance that some individuals get the timing right.