Parents do not pass genes one at a time to their children, and they pass whole 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. Which of a parent's two copies of a gene a child gets is chosen at random, and each copy has an equal chance. The odds that two brothers or sisters get the same versions of every gene is very small, and only identical twins share all of their genes. It is common to look more like one parent than the other, but that does not mean the child has genes from only that parent.

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 pea 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. In 1900 Hugo de Vries, Carl Correns and Erich von Tschermak rediscovered Mendel's work, years after his death in 1884.

Watson and Crick announced the double helix in the British journal Nature on 25 April 1953. DNA is two long strands that wind around each other in a spiral called a double helix, like a twisted ladder, and it is a code made of four chemical bases, A, G, C and T. The Human Genome Project ran from 1990 to 2003 and was one of the most ambitious scientific efforts in human history. 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. Some genes hold instructions for making proteins, which the body needs to work.

Eye colour depends on a region of chromosome 15 that holds two genes close together, OCA2 and HERC2, and it depends on how much of the pigment melanin is in the iris. These eye genes act like dimmer switches, and they turn melanin production up or down instead of simply on or off. 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. It is not common, but it happens. It is a myth that blue eyes are decided by a single gene. Eye colour comes from variation at several genes and the way they interact.

Scientists estimate that about 80 percent of a person's height is decided by differences in DNA, and food, health and activity decide much of the rest. By 2022 more than 700 gene variants linked to height had been found. As of November 2026, a 2022 study of 5.4 million people found 12,111 independent DNA variants linked to height that account for nearly all of the common-variant heritability. Traits shaped by several genes are called polygenic. They do not follow the simple patterns of Mendelian inheritance, and many are also shaped by the environment. How tall a person grows depends on the family they come from and also on what they eat and how healthy they are. Height is not set by one gene. There is usually no one-to-one match between a version of a gene and a trait. One variant rarely gives one fixed look.

Identical twins look very similar to each other and share all of their genes. Differences between identical twins come from the environment, since they share the same genes. 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. 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. Traits are shaped by genes and by environment together. Often the environment matters as much as genes, and sometimes more.

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. Identical twins share all of their genes, and non-identical twins share about half of the genes that differ between people, on average. Non-identical twins may look different, and may be a boy and a girl. Even identical twins have fingerprints that are not identical, and the details, such as where ridges meet, divide into branches or end, differ. A phenotype is a person's observable traits, such as height, eye colour and blood type.

Nearly every cell in a person's body has the same DNA, and inside each cell, DNA is wound into chromosomes. 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, and they do not rewrite the DNA letters. 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. Genes are sections of DNA, and a gene is the basic unit of heredity. Genes carry the instructions for traits such as eye colour and body type.

Kids often look a bit like the people in their family, but not exactly like any one of them. 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. Some things about a person come from how they live, not from their family: a child who eats well and stays active is likely to grow taller than a child with a poor diet. Where and how a child grows up, such as at home and at school, helps shape who the child is.

Words to know

• epigenetics: gene switching
• polygenic: many-gene
• pangenesis: Darwin's guess
• mitochondria: cell parts
• heritability: share from genes