THE MC TA TED GENE 1 1 3 



surface. The Deli, the deformis, and the hybrid plants begin 

 development with normal leaves; deformis forms early the char- 

 acteristic excrescences of the abnormal leaves. The hybrid 

 begins a few weeks later with the same process, and the Deli 

 race never does or does only later under certain conditions. 

 This, again, looks like a change of dominance, but it is easily 

 explained by the causation of chains of reactions of different 

 velocities leading to the incidence of the abnormality and having 

 three ascending velocities in the normal, the hybrid, and the 

 abnormal race. Honing also derives this explanation as a parallel 

 to the Lymantria case. 



Later, Honing (1928) reported another instance. There are 

 races of tobacco that need light for germination and races that 

 do not. In young seeds of hybrids between such races, the need 

 for light is dominant; in old seeds, recessive. 



Another comparable case has been described by Ferwerda 

 (1928) in the flour beetle (mealworm), Tenebrio molitor. Here a 

 pair of allelomorphs control orange and brown color through all 

 stages of life. In the hybrid, however, the larva and the pupa 

 are more orange; the imago, brown. It is easy to derive an 

 explanation on the basis of reaction velocities, considering the 

 facts regarding melanin formation (discussed in other chapters). 

 As a matter of fact, the case offers a close parallel to the develop- 

 ment of larval pigments in Lymantria. Ferwerda derives such 

 an explanation and constructs reaction curves for illustration. 

 If the product of reaction is the amount of chromogen produced, 

 the explanation derived for the pigmentation of Lymantria 

 caterpillars also fits this case completely under the assumption 

 that more chromogen is equivalent to deeper color. Other 

 interpretations of the same general type are easily imagined. 



D. Dominance and Chemistry of Products of Gene Action 



A few facts of a chemical nature are available that may one day 

 throw further light on the system of reactions within which the 

 quantitative shift is supposed to account for the facts of domi- 

 nance. We have already reported Schmalfuss' chemical models 

 of the oxidation of chromogens. It was shown that a number 

 of different factors influence the degree of this reaction, and it is 

 conceivable that, as far as melanin formation is concerned, the 

 model might be useful, showing that everything external, 



