You get your traits through a combination of the genes you inherit from your parents and the environment you grow up in. Every person receives one set of chromosomes from their mother and one from their father, creating a unique genetic blueprint that influences everything from eye color to height to disease risk. But genes alone don’t tell the whole story. Your diet, experiences, and surroundings also shape how those genes actually play out in your body.
DNA, Genes, and Chromosomes
Your body’s instruction manual is a molecule called DNA. It’s organized into roughly 20,000 protein-coding genes, and each gene carries instructions for building a specific protein or controlling a specific function. These genes are packaged into larger structures called chromosomes. Human cells carry 46 chromosomes, arranged in 23 pairs.
When your parents produced the egg and sperm cells that eventually became you, a special type of cell division called meiosis cut each parent’s chromosome count in half. The egg carried 23 chromosomes and the sperm carried 23. At fertilization, those two sets combined to give you a full set of 46, with one copy of each chromosome from each parent. This is why you share traits with both your mother and your father, but aren’t identical to either one.
How Dominant and Recessive Genes Work
For any given gene, you carry two versions, one from each parent. These versions are called alleles. Sometimes the two alleles you inherit are the same; sometimes they’re different. When they’re different, the interaction between them determines which trait shows up.
A dominant allele only needs one copy to produce its effect. If you inherit one dominant allele and one recessive allele, the dominant trait wins out. A recessive trait only appears when you inherit two recessive copies, one from each parent. This is why two brown-eyed parents can have a blue-eyed child: both parents can silently carry one recessive allele for blue eyes, and if the child inherits that recessive copy from both, the recessive trait appears.
Most Traits Aren’t That Simple
The dominant-versus-recessive model works neatly for a handful of traits, but most characteristics you can see or measure don’t follow such clean rules. Height, skin color, body weight, and intelligence are all polygenic traits, meaning they’re influenced by dozens, hundreds, or even thousands of genes working together. Each individual gene contributes a small effect, and all those effects add up.
Eye color is a good example of this complexity. For a long time, it was taught as a simple dominant-recessive trait. In reality, at least two major genes and several smaller ones interact to determine your eye color. A recent large-scale genetic study identified 50 distinct locations in the genome associated with eye color variation, including genes involved in pigment production and the physical structure of the iris itself. That’s why eye color exists on such a wide spectrum, from deep brown to hazel to green to pale blue, rather than falling into just two or three categories.
Why Siblings Look Different
If you and your siblings inherited genes from the same two parents, you might wonder why you don’t look identical. The answer lies in how egg and sperm cells are made. During meiosis, two things happen that shuffle the genetic deck.
First, your 23 chromosome pairs line up randomly before splitting apart. The copy of chromosome 1 you pass on has no influence over which copy of chromosome 2 gets included. With 23 pairs sorting independently, a single person can produce about 8 million different chromosome combinations in their egg or sperm cells. Second, before the chromosomes separate, they physically swap segments with their partner in a process called crossing over. This recombines genetic material within individual chromosomes, creating even more variation. When you combine the possibilities from both parents, the number of unique genetic outcomes for any given child is essentially astronomical. That’s why siblings can share the same parents yet look and act noticeably different from each other.
Traits That Come From Neither Parent
Occasionally, a child displays a trait that neither parent has and that doesn’t run in the family at all. This can happen through a de novo mutation, a spontaneous change in DNA that occurs for the first time in a sperm cell, an egg cell, or in the fertilized egg shortly after conception. De novo mutations are one way genetic conditions can appear in a family with no prior history of them. They’re also, over many generations, one of the engines of evolution, introducing entirely new genetic variation into a population.
How Your Environment Shapes Your Genes
Your DNA sequence is fixed at conception, but how actively your body uses each gene can change throughout your life. This is the field of epigenetics. Your behaviors and environment can attach or remove small chemical tags on your DNA. One of the most common tags is a methyl group. When a methyl group attaches to a gene, it typically dials that gene’s activity down or silences it entirely. When the tag is removed, the gene becomes active again. These changes don’t alter the DNA code itself, but they change how much protein a gene produces, which can affect your traits, your health, and even your risk for disease.
Diet, stress, physical activity, exposure to pollutants, and many other environmental factors can trigger these epigenetic changes. This is one reason why identical twins, who share the same DNA, can develop different health conditions or look slightly different as they age. Their environments gradually dial different genes up or down.
The Balance Between Genes and Environment
Scientists use a measure called heritability to estimate how much of the variation in a trait across a population is driven by genetics versus environment. Heritability doesn’t tell you how much of your personal height came from genes. Instead, it describes how much of the differences in height between people in a group can be attributed to genetic differences.
Height is one of the most heritable common traits, with genetics explaining roughly 60% or more of the variation between people. Intelligence shows an interesting pattern: its heritability starts at around 20% in infancy and climbs to as high as 80% in later adulthood, meaning genetic influences on cognitive ability become more pronounced as you age and increasingly select your own environments. Personality traits are less heritable, with genetic factors explaining a smaller share of the differences between people.
For many of the most important health conditions, including cancer, heart disease, and diabetes, the genetic component comes from the combined small effects of many genes interacting with lifestyle and environmental factors. This is why having a family history of a disease raises your risk but doesn’t guarantee you’ll develop it, and why people with no family history can still be affected.
Predicting Traits From DNA
With advances in genetics, researchers have developed tools called polygenic risk scores that add up the effects of thousands of small genetic variants to estimate a person’s predisposition toward a trait or disease. In theory, if every genetic effect were measured perfectly, these scores could predict a trait as well as the heritability of that trait allows. In practice, the predictions fall well short.
For body mass index, one of the better-performing scores explains only about 5% of the variation between individuals. People with the highest genetic risk scores do show elevated rates of severe obesity, but most people who are obese have average or even low genetic risk scores. The uncertainty in any individual prediction remains extremely large. These tools are useful for research and for identifying population-level trends, but they can’t reliably tell any one person what traits they’ll have or what diseases they’ll develop. The interplay between thousands of genes and a lifetime of environmental influences is simply too complex to reduce to a single number.