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




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.


The relation between the seed weight of the parental generation and the filial generation in a variety of brown beans.

The figures in the table represent the filial numbers of beans of the different weight categories

weight of the parental beans

weight of the filial beans
weight categories
 
10
20
30
40
50
60
70
80
90

average

20
-
1
15
90
63
11
-
-
-
43,8
30
-
15
85
322
310
91
2
-
-
44,5
40
5
17
175
776
956
283
24
3
-
44,2
50
-
4
57
305
521
196
51
4
-
48,9
60
-
1
23
130
230
168
46
15
2
51,9
70
-
-
5
53
175
180
64
15
2
56,0
total
5
38
170
1676

2255

928
187
33
2
47,92
according to W. JOHANNSEN; 1903, 1926


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).



© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de