100 



I'llYSloUHlKM. UENETICS 



general way by the words: The faster reaction wins the race. 

 The further development of genetical knowledge, especially of 



the interaction of genes, the action of so-called modifiers, led to 

 a general recognition of the idea that dominance is not a property 

 of the gene but a phenotypic result of the action of the gene in 

 connection with other gene-controlled developmental processes 

 or environmental actions. Wright (1925), in connection with 

 the fact that the Wild-type gene C is dominant over all members 

 of the albino series, designated such a process a threshold 

 effect, controlled by other genes. A generalized theory of 

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Fig. 23. — Diagrammatic representation of dominance relations if the number 

 of cell divisions determines the phenotype, if and the effect of the heterozygote 

 is intermediate between that of the homozygotes. M, M\, J/2 are different sys- 

 tems of incidence of cell division. {From Goldschmidt, 1927. Phys. Th. d. Ver.) 



dominance as a phenomenon of development was developed by 

 Goldschmidt (1927c), who explained the phenomenon as a neces- 

 sary consequence of the type of action of the gene in controlling 

 development. This theory is still the only one adaptable to all 

 facts and the only one that furnishes the necessary physiological 

 basis to all phylogenetic theories of dominance (see page 121). 

 The following case may serve as a model which might be adapted 

 easily to other cases by changing the respective variables. Let us 

 assume two allelomorphs controlling a character which might be 

 measured in quantitative terms, like the number of facets in an 

 insect eye. The dominant gene controls a reaction of a certain 

 velocity; the recessive allele, a reaction of lower velocity; and the 

 heterozygote, an intermediate reaction (see Fig. 23). The 

 reaction leads to a determining effect when the products have 



