Round 8: The Johannsen Breakthrough - Pure-Line Theory (1903), Gene and Genotype–Phenotype Distinction (1909)

In 1903, JohannsenJohannsen, Wilhem performed a key experiment that had a significant influence in the biometry-saltationismbiometry-saltationism debate debate. He planted seeds from the brown “Princess”Princess beans beans and grouped the F1 offspring according to the weight classes of the parent seeds. He found that, as the law of regression predicted, the deviation in the mean weight of a particular group of offspring from the mean F1 weight was less extreme than the deviation in the weight class of the parents from the parental mean. However, when JohannsenJohannsen, Wilhem divided the beans according to pure lines (i.e., different homogenous groups), the offspring in a given pure line showed complete regression to their own mean, irrespective of the deviations of their parents. This showed that natural selectionselectionnatural was ineffective in pure lines, i.e., was unable to act on individual differencesindividual differences. In 1909, JohannsenJohannsen, Wilhem introduced the terms phenotype, gene, and genotype, and made the genotype–phenotype distinction.