162 PHYSIOLOGICAL GENETICS 



spite of the immense literature upon simple recombinations of 

 genie effects) if thoroughly investigated, because it is highly 

 probable that somehow a change in developmental reactions 

 caused by a mutant gene will create a situation that affects also 

 the workings of simultaneous or consecutive processes. 



2. All transitions will be found from such a situation to the 

 other extreme, viz., that the reactions caused by different genes 

 act directly or indirectly upon the same process. An indirect 

 action would occur, for example, if one gene-controlled reaction 

 changed the pH value present when the other gene-controlled 

 reaction occurred. A direct action might be of various types as 

 follows: 



a. One reaction produces a substance necessary for some 

 morphogenetic process; the other, another substance with the 

 same type of action, e.g., two different types of growth hormone. 



b. Both reactions affect different developmental processes, 

 involved in the formation of the phenotype; e.g., one increases the 

 rate of cell division; the other, cell growth; and their effect upon 

 size is therefore additive. 



c. Both reactions have an identical effect, reached possibly by 

 different means; e.g., a definite substance is as well produced by 

 oxidation of something as by reduction of something else. The 

 result again would be additive. 



d. The different genie reactions affect different phases of 

 general developmental processes; e.g., one changes growth in 

 length; the other, growth in width; the result might be additive, 

 but it might also be a compromise between both effects. 



Other possibilities may easily be visualized. In a general way, 

 we may therefore expect interactions to be either combinatory or 

 additional or of the nature of a compromise or of a combination 

 of these three possibilities. The actual effect will depend upon 

 all those variables of gene-controlled reactions that have already 

 been described, viz., velocity of the main reaction, threshold 

 values, time of final determination, time of onset of different 

 decisive reactions. Examples for such interactions may therefore 

 be found at many places in former chapters. 



A systematic attack upon these problems has been made also in 

 Drosophila. Schultz (1929) used the so-called Minute dominant 

 mutations. (As a matter of fact, many if not all of these are 

 actually deficiencies; for the present discussion, the nature of the 



