The odds that two brothers or sisters get the same versions of every gene is very small. Only identical twins share all of their genes. Brothers and sisters from the same two parents each get a different mix, so they are not exactly alike.

Parents do not pass genes one at a time to their children. They pass whole chromosomes. A person normally has 23 pairs of chromosomes, 46 in all. Which of a parent's two copies of a gene a child gets is chosen at random, and each copy has an equal chance.

It is a myth that blue eyes are decided by a single gene. Eye colour depends on a region of chromosome 15 that holds two genes close together, OCA2 and HERC2. Several other genes play smaller roles. Parents with blue eyes can have a child with brown eyes. It is not common, but it happens.

Eye colour depends on how much of the pigment melanin is in the iris. Eye colour comes from variation at several genes and the way they interact. There is usually no one-to-one match between a version of a gene and a trait. One variant rarely gives one fixed look.

The human genome has about 19,900 genes that make proteins, as of November 2026. DNA is a code made of four chemical bases, A, G, C and T. Human DNA has about 3 billion bases. More than 99 percent of the DNA bases are the same in all people.

Height is not set by one gene. It is determined by multiple gene variants, an inheritance pattern called polygenic inheritance. Scientists estimate that about 80 percent of a person's height is decided by differences in DNA. Food, health and activity decide much of the rest.

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. How tall a person grows depends on the family they come from and also on what they eat and how healthy they are.

Studies of twins compare how alike identical twins are with how alike non-identical twins are. The comparison separates the effects of genes (nature) and environment (nurture). Identical twins share all of their genes. Non-identical twins share about half of the genes that differ between people, on average.

Even identical twins have fingerprints that are not identical. The details, such as where ridges meet, divide into branches or end, differ. 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. Looks can come from family and from surroundings together. Not everything about a person is decided by the body's instructions alone.

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. Mendel's pea plants suited controlled breeding, and one plant gave him hundreds of peas with traits he could see.

Some children join a family by adoption. 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.

Words to know

• chromosome: wound-up DNA
• melanin: colour pigment
• polygenic: many-gene
• genome: all the DNA
• variants: gene versions