More support for the Mendelian law of inheritance was provided by Johannsen, a Danish botanist, who showed that in certain self-fertilizing plants a pure line of descendants can be maintained indefinitely, in which case natural selection is not effective, selection depending upon genetic variability. He introduced the term “gene” in 1909.

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).