Genetics was born in the first years of the twentieth century with the discovery that hereditary characters are carried by molecules that are physically present in chromosomes. These molecules of heredity – the genes – are responsible for the visible structures of the organisms but do not enter into those structures, which means that in every cell there are molecules which determine the characteristics of other molecules. In 1909, Wilhelm Johannsen concluded that this distinction is similar to the difference which exists between a project and its physical implementation, and represents therefore a dichotomy of the living world which is as deep as the Cartesian dichotomy between mind and body.

In order to distinguish the two types of molecules Johannsen called them genotype and phenotype, but such a dualism was almost universally rejected. At that time it was thought that proteins were responsible for both the visible structures and the hereditary characters, and all biological features seemed reducible to a single type of molecule. The reality of the genotype–phenotype distinction was proved only in the 1940s and 1950s, when molecular biology discovered that genes are chemically different from proteins, and, above all, when it became clear that genes carry linear information whereas proteins function through their three–dimensional structure.

The genotype–phenotype duality is therefore a dichotomy which divides not only two different biological functions (heredity and metabolism), but also two different physical quantities (information and energy).