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The Behaviour of Genes in Populations.
The Hardy-Weinberg Equilibrium

• The Hardy-Weinberg Equilibrium
• The Analysis of Quantitative Data
• Literature

The experiments performed by W. JOHANNSEN with pure lines of Phaseolus vulgaris allowed the distinction between genotype and the effects of environmental factors on the genotype that result together in the specific phenotype of an organism. Though variations caused by the environment may lead to differences in the phenotype of a pure line, a selection within such lines stays without effect. These experiments were necessary preconditions for the theoretical and statistical evaluation of allele frequencies in populations.

Variation of seed size in a pure line population of Phaseolus vulgaris (H. de VRIES, 1906)

An important precondition for the attainment of reproducible results is the use of defined source material. Pure lines are not always available from the start.

Perfectly independent of the attempts talked about up until now are the studies of the Dane W. JOHANNSEN who analyzed the variability of the french bean (a variety of Phaseolus vulgaris). Numerous pure lines that differ in certain properties like the average seed weight exist in this obligatorily self-pollinating variety. These differences are determined genetically and thus an element of the genotype. But for a number of reasons, like, for example, the position of the pod at the plant and the resulting differences in the supply with assimilates and other nutriments, every plant produces seeds of different weight. The arrangement of the seeds that is caused by extern factors represents an element of the phenotype that is achieved by a combination of hereditary and environmental factors.

JOHANNSEN chose the lightest and the heaviest seeds of the phenotype-variations of pure lines of several subsequent generations for cultivation without achieving a change of the average seed weight. A selection within pure lines is thus without effect (see table). Based on these findings did JOHANNSEN coin the terms genotype and phenotype.

Variations caused by the environment have also to be taken into account when evaluating crossings. An example will illustrate this. E. M. EAST (1910) crossed a variety with long cobs with varieties with short cobs when studying the inheritance of the cob length of maize. The F₁ was intermediate but not strictly uniform. The F₂-generation displayed far greater variations since the influences of environmental factors and the occurrence of different genotypes overlapped. In such cases, it is impossible to identify the single genotypes directly.

Inheritance of a quantitative character in maize. Cob length of the varieties Tom Thumb (P₁), Black Mexican (P₂) and the hybrids (according to E. M. EAST, 1910).