Every person has two copies of each gene, one inherited from each parent.
Most genes are the same in all people, but a small number of genes (fewer than one per cent) are slightly different between people. These differences contribute to the uniqueness of a person, for example, height or hair colour.
While most variations in a person’s DNA do not have an effect on health, sometimes variations or “spelling mistakes” can mean that the gene where that variation occurred is no longer able to work correctly. A condition caused by variations in one or more genes that disrupts normal development or causes a medical condition is called a genetic disorder.