THE MUTATED GENE 73 



which led to a generalization of the idea. Wright found a 

 serievS of allelomorphs of the albino type in guinea pigs, which 

 produced an orderly effect upon skin color and which could be 

 seriated. The general nature of the effects suggested that the 

 albino series has to do simply with the rate of a process that is 

 essential to all melanin production. But a difficulty was met 

 when the effects of the series of genes were studied in relation 

 to the underlying constitution of black fur, because it was found 

 that the order of effects was not the same. But this might have 

 been due to secondary processes in the physiology of pigment 

 production, e.g., threshold conditions. We shall return to this 

 special point in a later chapter (see page 88), where the details 

 of the case will be reported. Here it suffices to say that a later, 

 more complete analysis (Wright, 1925) led the author to confirm 

 his earlier views that the factors of the albino series determine the 

 rate of some one process fundamental to all pigmentation. 



In Drosophila, Plunkett (1926) first applied the concept of 

 gene-controlled rates to the results of his experiments. The 

 school of Zeleny (see page 75) had already shown that a gene- 

 controlled character, the number of facets in the Bar eye, varies 

 reciprocally as the temperature at which the flies were raised. 

 They expressed the results in terms of the temperature coefficient 

 for the rate of facet formation but did not conclude that the 

 mutant gene itself acts by changing the rate of some process, as 

 Wright and Goldschmidt had done. Plunkett used for this 

 experiment the gene Dichaete which removes certain bristles on 

 the thorax of Drosophila, especially the dorsocentrals. This 

 effect, as usual, is influenced by modifiers and external conditions, 

 and it reacts most regularly to temperature changes and, accord- 

 ing to Plunkett, without a sensitive period. Table 6 gives the 

 data for mean numbers of the posterior dorsocentral bristles on 

 one side of the body (1 in the Wild type). 



Plunkett analyzed these data mathematically. As only a 

 single bristle is involved which is produced at a definite time, the 

 concentration of a bristle-forming or a bristle-suppressing sub- 

 stance must be involved. 1 The effect may then be produced 

 either by an increase, relative to other developmental processes, 

 in the rate of a reaction, breaking down a bristle-forming sub- 



1 It is not known whether these mutants prevent bristle formation or 

 destroy already formed bristles, as is the case in vestigial wings. 



