DNA is a code made of four chemical bases, A, G, C and T, and human DNA has about 3 billion bases, wound into two long strands that wind around each other in a spiral called a double helix, like a twisted ladder. DNA is the molecule that carries genetic information, and nearly every cell in a person's body has the same DNA. Genes are sections of DNA, and some genes hold instructions for making proteins, which the body needs to work. The human genome has about 19,900 genes that make proteins, as of November 2026, and the information for all of them sits in only about 1.5 percent of the human genome. More than 99 percent of the DNA bases are the same in all people. The Human Genome Project ran from 1990 to 2003 and was one of the most ambitious scientific efforts in human history, and in 2003 it produced a genome sequence that accounted for over 90 percent of the human genome.
Inside each cell, DNA is wound into chromosomes, and a person normally has 23 pairs of chromosomes, 46 in all. People have two copies of each gene and inherited one copy from each biological parent, so half of a person's genes come from each. Genes come in different versions called alleles, and when the two alleles of a gene differ, one allele (the dominant one) is expressed, and the effect of the other (the recessive one) is masked. Parents do not pass genes one at a time to their children. They pass whole chromosomes, and which of a parent's two copies of a gene a child gets is chosen at random, with each copy having an equal chance. The odds that two brothers or sisters get the same versions of every gene is very small, and brothers and sisters from the same two parents each get a different mix, so they are not exactly alike.
Eye colour is more complex than one gene, and many genes work together to decide it. Eye colour depends on how much of the pigment melanin is in the iris, and it depends on a region of chromosome 15 that holds two genes close together, OCA2 and HERC2. These eye genes act like dimmer switches. They turn melanin production up or down instead of simply on or off. It is a myth that blue eyes are decided by a single gene, and many people think two blue-eyed parents cannot have a brown-eyed child, but the simple one-gene idea behind this was first suggested in 1907 and does not hold up. Parents with blue eyes can have a child with brown eyes, which is not common, but happens. There is usually no one-to-one match between a version of a gene and a trait.
Genetics plays a critical role in the size and shape of our facial features, and nearly all studies of faces report high heritability, more than 60 percent, for at least some facial traits. Studies of soft tissue find the highest heritability for aspects of nose shape, and non-genetic factors such as diet, climate and the forces of chewing also help shape the face. Identical twins raised apart have facial features that are as alike as identical twins raised together, which suggests the environment has only a limited effect on the face. Identical twins share all of their genes, and non-identical twins share about half of the genes that differ between people, on average. Studies of twins compare how alike identical twins are with how alike non-identical twins are, to separate the effects of genes (nature) and environment (nurture). Identical and non-identical twins both grow up in the same home at the same age, so the two kinds differ mainly in how many genes they share. Even identical twins have fingerprints that are not identical. The details, such as where ridges meet, divide into branches or end, differ.
Height is not set by one gene. It is determined by multiple gene variants, an inheritance pattern called polygenic inheritance, and traits shaped by several genes do not follow the simple patterns of Mendelian inheritance. Scientists estimate that about 80 percent of a person's height is decided by differences in DNA. By 2022 more than 700 gene variants linked to height had been found, and a 2022 study of 5.4 million people found 12,111 common variants linked to height. As of November 2026, common DNA variants predict about 40 to 50 percent of the differences in human height between people. Missing heritability is the gap between heritability estimated from DNA data and heritability estimated from twin data. Around 2010 only about 40 gene variants for height were known, explaining about 5 percent of the variation, far below the twin estimates. A 2010 study found that about 45 percent of the variation in height could be explained by the genetic variation captured on a genotyping array. As of November 2026, some scientists ask whether twin studies have overestimated heritability for human traits. This is still discussed.
Mitochondria are parts of the cell that have their own DNA, unlike other cell parts, and children inherit their mitochondria, and so their mitochondrial DNA, from their mother. Epigenetics is the study of how cells control gene activity without changing the DNA sequence, and epigenetic changes are modifications to DNA that turn genes on or off. They do not rewrite the DNA letters. A person's diet and exposure to pollutants can affect the epigenome. As of November 2026, whether epigenetic marks acquired during a life are passed on to children is debated, and in humans the evidence is limited, and scientists say the human evidence is virtually nonexistent so far. As of November 2026, findings from human groups that suggest effects across generations still await independent replication, so they should be read with caution.
Charles Darwin knew that traits had to be inheritable for species to change, but when he wrote On the Origin of Species he had no explanation for how inheritance worked. In 1868 Darwin published a guess, his hypothesis of pangenesis, which said heredity worked through tiny particles he called gemmules. Gregor Mendel worked out basic laws of inheritance by studying pea plants, and his plants suited controlled breeding, and one plant gave him hundreds of peas with traits he could see. Mendel's work was not recognized until after his death in 1884, and he did not use the words gene or genetics, but in 1900 Hugo de Vries, Carl Correns and Erich von Tschermak rediscovered it. A 1944 paper by Avery and McCarty and colleagues concluded that DNA was the substance causing a lasting heritable change in bacteria. Rosalind Franklin's team took the X-ray picture called Photograph 51 at King's College London in May 1952, and it showed a pattern of spots highly suggestive of a helical structure. Watson and Crick announced the double helix in the British journal Nature on 25 April 1953, and nine years after that paper, Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine.
Some children join a family by adoption, and an adopted child becomes a full and permanent member of that family, with the same legal rights as a child born into the family. A blended family, also called a stepfamily, forms when two people make a life together with their children. Heredity is how traits get passed down from parents to their children. Traits are shaped by genes and by environment together, and often the environment matters as much as genes, and sometimes more. Where and how a child grows up, such as at home and at school, helps shape who the child is.
Words to know
- heritability: share from genes
- replication: repeat test
- genotyping: reading DNA
- alleles: gene versions
- epigenome: gene switches
