The usual idea of a gene is of a specific region of DNA that codes for a single protein or enzyme, and the position of a gene on a chromosome is known as its locus. Variants of the DNA sequence at this locus among individuals are known as alleles. This ‘one gene, one enzyme’ model has long been the basis for research by human geneticists trying to identify traits, or phenotypes, whose inheritance patterns are consistent with the action of individual genes.

However, recent advances in genetics have greatly revised our concept of what genes are and how they work, showing that the relationship between DNA sequence and phenotype is both more complex and more interesting than we had thought. Some functions of DNA do not even depend on its nucleotide sequence, and DNA sequence variation includes a variety of direct and indirect forms of feedback among various regions of the DNA within and between cells.

Human life begins with a fertilized egg that carries a set of chromosomes containing genetic instructions, plus enough basic compounds and nutrients to commence the cell cycle. This latter material includes messenger RNA (mRNA) coded for by the mother's genes, which provides enough ‘information’ to direct the development of the new individual until its own genetic mechanism can be switched on. Thereafter, the organism depends entirely on its own genes, which contain the coding sequences and all the other signals needed for controlling embryological development, and the subsequent growth, tissue renewal, and physiology of the organism during its lifetime.