THE MUTATED GENE 65 



mechanics of the case are unknown. A zoologist would suspect 

 that the normal differentiation of sepals and petals has to do 

 with a process that we shall later discuss as stratification of 

 determining substances (which occurs a long time before visible 

 differentiation). Sepalody would then be the effect of insufficient 

 segregation of such stuffs in the flower bud. If this were the case, 

 an interpretation parallel to the wjr-case involving velocities of 

 definite processes might be developed easily. Unfortunately, the 

 physiological facts are lacking. But there is one significant fact 

 strongly pointing in the direction of such a general explanation: 

 Oehlkers (1935) found that the Cruciata gene increases the rate 

 of differentiation of the plant and made it even probable that this 

 increase is proportional to the relative strength of these 

 allelomorphs. 



2. The Threshold Concept. — Before reporting further cases 

 of the same type, we must consider the threshold concept, which 

 we are facing here for the first time and which will reappear on 

 many occasions. The diagram (Fig. 20) shows in the curve for 

 the Wild-type wing (on top) that the production of the decisive 

 substance always has to be above the necessary minimum, the 

 threshold, to insure normal development (or, in the reciprocal 

 conception, below the threshold). We could imagine that many 

 more such curves above the normal curve exist, all of which 

 would be above the threshold and therefore control different 

 degrees of Wild-type, viz., plus minus hyperwild. We shall return 

 to this special point in the chapter on dominance and shall discuss 

 here only the general features of the threshold concept. Early 

 in the development of the theory of genie action by rates of 

 reaction, it became clear that the explanation of many details 

 requires the assumption that the products of those reactions 

 which might generally be called determining stuffs act only when 

 they have reached a minimum concentration, i.e., a threshold 

 level. To this may be added the conception that there is also a 

 maximum of response: Above a certain level of concentration, no 

 increase in response is possible; e.g., an increase in the wing- 

 growth substances will not change the effect, the maximum of 

 which is the Wild-type wing. This level of possible maximum 

 response may coincide with the minimum threshold, as in the 

 case just mentioned, or it might also be higher. We shall meet 

 later with examples. 



