DEVELOPMENT OF THE HETEROSIS CONCEPT 65 



when selfing reduced the necessary genes below a stage needed by the or- 

 ganism. The following quotation from Thorssell emphasizes the viewpoint 

 that the relative importance of genes controlling heterosis is greatly in- 

 fluenced by other factors of the organism: 



The cumulative effect of heritable characters, however, brings it about that develop- 

 ment, that is to say green weight, does not stand in arithmetical proportion to the number 

 of pairs of the dominant genes in question. From this it follows also that the said number can 

 be reduced within a certain limit without perceptible or any great influence upon green 

 weight. If this limit is exceeded, a considerable degeneration sets in. 



The speaker has chosen to consider heterosis as the normal expression of 

 a complex character when the genes concerned are in a highly heterozygous 

 condition. x'Vs most normal characters are the end result of the action, reac- 

 tion, and interaction of countless numbers of genes, and as gene mutation 

 constantly occurs although relatively infrequently, it may be impossible to 

 obtain all essential genes in the most favorable homozygous state. After 

 selecting the best homozygous combinations, further vigor will be obtained 

 due to heterozygous combinations of factors. Dominance or partial domi- 

 nance seems of great importance as an explanation of hybrid vigor. In some 

 cases there may be extra vigor correlated with the heterozygous condition of 

 pairs of alleles. The types of response of inter and intra allelic factor interac- 

 tions are without doubt dependent upon both external and internal environ- 

 ment. 



