A century ago, “genes” endowed with the instrumental reductionist status of the Mendelian Faktoren were introduced to overcome the notion of the unit character. The gene was an empiric, functionally defined entity. For Johannsen, gene was a word derived from genotype that should express “only the simple imaging” that “through ‘something’ in the gametes a property of the developing organism is conditioned or is, or could be co-determined” (Johannsen, 1909, 124).

Yet, the meaning of the gene, whether as an intervening variable, a heuristic device for the study of inheritance, or a hypothetical construct, an entity in the physico-chemical world, and what kind of such an entity, became a major issue of genetic analysis. From Richard Goldschmidt's “physiological” perspective genes were merely centers of gravity along the integral units of the chromosome. For Hermann Muller genes were discrete structural entities along the chromosomes. Others, like East, Morgan, and Stadler adopted an operational approach of “instrumental reductionism” (Falk, 1986, 141).