THE MUTATED GENE 119 



is always an addition of the actions of the individual genes or, 



in other words, always intermediate. The fact that Wild type 



is completely dominant in certain combinations shows that the 



Wild-type gene has in homozygous condition an action far 



beyond the necessary threshold (see Fig. 20) ; furthermore, that 



one Wild-type gene is not dominant but acts only as a dominant 



if the partner in the heterozygote has an action of such a quantity 



A -4- a 

 that — n — reaches the threshold. If, however, this partner, 



as in the No-wing/ -f- heterozygote has itself such a low quantity 

 of action that the quantity of the heterozygote falls below the 

 threshold, Wild type is no longer dominant, and the lower mem- 

 ber of the series is called a partial dominant. 



Just as the correctness of these conclusions for the right 

 end of the series could be proved by the parallel shifting action 

 of the dominigenes, so it can be proved also near the threshold 

 point by the action of the dominigenes upon the No- wing/ + 

 heterozygote, which is shifted into 100 per cent scalloping. 



3. The analysis of dominance in this case can be pushed one 

 step further. In Fig. 20 (page 59), we presented the interpreta- 

 tion of the ygf-allelomorphs in terms of reaction velocities, based 

 upon the fact that with increase of the effect (destruction of 

 wing area) the onset of the process of destruction occurs earlier 

 and earlier in development. If we follow here only the interpreta- 

 tion for insufririeney of a growth substance, which might easily 

 be translated into the one for lysis, the topmost curve in this 

 figure represents the production of the growth substance just 

 above the threshold for Wild type. Comparing this figure with 

 Mohr's table on page 118, this curve would correspond to the 

 value 40 of the table. The curve for Wild type would be far 

 above this figure in the direction of the arrow, marked hyper- 

 irild. As we know that the curve for heterozygous No-wing 

 is similar to the nicked curve in the diagram, and as we have to 

 assume that, ceteris paribus, the curve of the heterozygote is 

 exactly between those of the homozygotes, the Wild-type curve 

 would have to be drawn as far above the m-curve as the A%-curve 

 is situated below it. Thus we realize that in this case dominance 

 is based upon a system of developmental reactions, as described 

 in our model (Fig. 23) ; the heterozygous gene produces an inter- 

 mediate reaction which leads to a phenotypically intermediate 



