Pure Lines

PURE LINES, a term given by Johannsen to the progeny of a single self-fertilizing, absolutely homozygous individual. It is assumed here that all descendants of this individual reproduce solely by self-fertilization. All individuals making up a single pure line are absolutely homozygous (for all genes) and possess an identical genotype. Phenotypic differences within a pure line represent fluctuations (see), i.e., changes not associated with changes in genes. As Johannsen's classical studies on pure lines in beans, as well as studies by a number of authors on other objects, have shown, selection within a pure line is impossible. Johannsen selected extreme variants within a pure line for a number of generations, but their progeny turned out, as a rule, to be identical. Only the emergence of a mutation (see) within a pure line provides material for selection. In the very next generations after the emergence of a mutation in self-pollinating plants, it passes into a homozygous state and selection again becomes impotent. Having proven the absence of the effect of selection in pure lines, Johannsen thereby refuted the so-called Galton's law of regression. Galton, using statistical analysis, showed that parents deviating from the mean value of the "race" give progeny that also deviate (although to a lesser degree) in the same direction as the parents from this mean. He spoke of the fact that the properties of children exhibit a regression of the individual characteristics of the parents. Johannsen showed the error of Galton's observations, who was dealing with genetically heterogeneous (heterozygous) material. Within a pure line, as Johannsen showed, the deviation of parents from the mean value of the "race" is not inherited at all. Johannsen's refutation of the inheritance of individual deviations caused by external conditions dealt a decisive blow to the views of the Lamarckians. Even before Johannsen, the practical horticulturist Louis Lévêque de Vilmorin introduced into practice the individual testing of the progeny of each plant, isolating it from the progeny of other plants. Vilmorin spoke of the "different hereditary power of plants." Johannsen proposed the name "Vilmorin's principle" for the principle of individual testing of the descendants of each plant, under which Vilmorin was dealing with pure lines according to Johannsen's terminology.