Elemente der exacten Erblichkeitslehre.

In this work Johannsen coined the term “gene” as the “underlying structure in the organism, that which was transmitted during hybridzation.” He also coined the term "phenotype" to express what is actually observed and can be measured, in contrast to "genotype" that he coined “to express the underlying constitution of the organism from which development of the organism begins" (Brock).