Changes of Heritable Tendencies in Pure Lines and Populations

Abstract

The distinction Johannsen made in 1903 between what he called pure lines (reine Linien) and populations,was indeed one of the sharpest theoretical distinctions ever made between concepts in genetics, in its struggle to become a more exact science. It is true that his definition of the ‘pure line’ concept changed somewhat in the course of the years. In 1903 (p. 9) he formulated the concept as ‘individuals which descend from one single self-fertilized individual’, in 1909 (p.133; 1926, p. 178) as ‘all individuals which descend from a single absolutely self-fertilized homozygous individual’ and in 1915 (p. 606): ‘a pure line comprises all descendants of a single absolutely self-fertilizing individual which does not have a hybrid nature of its own’. However the 1903 description of the concept only became more detailed and exact by these alterations. The individuals of a pure line are all genetically identical when compared with the parental individual. On the other hand a population is a mixture of individuals which are different in heritable tendencies. These differences can lie in the fact that some factors are homozygously present in all individuals and the remainder heterozygously present. It is obvious that such a situation can be found in a population of a district or region as well as within a taxonomical group (species or race) of plants or animals.