How did Wilhelm Johannsen clarify the difference between traits and genes?

Early geneticists, including Gregor Mendel, did not consistently distinguish between an organism's observable characteristics and the underlying hereditary determinants responsible for them.1

In 1909, Danish botanist Wilhelm Johannsen clarified this distinction by introducing two fundamental terms that remain central to genetics:21

• Genotype: Johannsen used this term to describe the total genetic inheritance or the heritable units of an organism.23
• Phenotype: He introduced this term to describe the observable and measurable physical or biochemical characteristics of an organism, which result from the interaction between the genotype and the environment.23

Johannsen also coined the term genes to specifically identify the hereditary determinants themselves. Through his experiments with "pure lines" of beans, he demonstrated that individuals could exhibit different phenotypes (such as varying sizes) while possessing identical genotypes, proving that external traits are not always direct reflections of the underlying genetic material.13