In 1903 a Danish biologist, Wilhelm Johannsen, reported an important finding that has
provided the foundation for modern breeding methods. He showed that progeny grown
from a single plant selected from a mixture of inbred lines would produce progeny that
were consistently different from those of another plant from the same mixture. Thus, he
could create a large-seeded variety and a small-seeded variety through single-plant selec-
tions from the same mixture. Importantly, he also observed that further selections within
progeny that were derived from the same single plant were not effective. This is because
each selection represented a pure homozygous line, and all subsequent variation observed
within a selected line was due to differences in environment, and not to genetic differences.
A variety selected and multiplied from a single homozygous plant is known as a “pure line,”
and the alleles or traits possessed by this line are said to be “fixed,” meaning that further
selection is neither necessary nor possible. These observations, as illustrated in
Figure 3.3, are known asJohannsen’s pure-line theory. It is also noteworthy that these
observations were probably the first time that a clear distinction was made between
genotype and phenotype—an important step beyond Mendel’s laws.
Figure 3.3.Development of pure lines from a mixture of homozygous, heterogeneous beans. Panels
(a) through (d) show histograms representing the frequency of different bean sizes in various popu-
lations. Panel (a) could represent a landrace or a population derived by repeated selfing of progeny
from a hybridized plant. Selection of single beans from the original population results in new popu-
lations (b) that have different average bean sizes. Further selection within these populations is not
effective (c, d). In this illustration bean color is a qualitative trait that shows no environmental vari-
ation, whereas seed size is a quantitative trait that shows environmental variation. The dark-colored
beans on the right represent a pure line, and all phenotypic variation for seed size within that line
is environmental. A Danish biologist, Wilhelm Johannsen, conducted a similar series of experiments
in 1903, and developed what we now call thepure-line theory.
54 PLANT BREEDING