364 Action of the Genetic Material 



partially dominant efiFect of a mutant locus in relation to all the devel- 

 opmental reactions involved in the final effect of the genie action, 

 with the help of a model (see fig. 18), which, I think, can be used 

 for all aspects of the problem. Let us assume we are dealing with a 

 pair of alleles controlling a character which can be measured (e.g., 

 the number of facets in an insect eye). The dominant locus controls 

 a reaction of a certain velocity, expressed in the diagram as the eleva- 

 tion of a curve (for simplicity's sake represented as a straight line). 

 The recessive allele controls a lower velocity; and the heterozygote, 



Fig. 18. Diagram explaining dominance in terms of reaction velocities and 

 "buflFering." (From Goldschmidt, 1927.) 



an intermediate one. The products of reaction attain a determining 

 effect when they reach a certain concentration, the threshold level 

 M. In order to have a very simple model, we assume that the heredi- 

 tary character is the size of some structure attained by the respective 

 number of cell divisions and that the determining stuff is one that 

 puts an end to further division. Of course, a similar model could be 

 constructed with different assumptions. If the progress of cell division 

 up to the point of stoppage is determined independently, the size 

 of the organ will be smaller the earlier the curve reaches the level M. 

 If all cell divisions proceed in equal intervals, the heterozygote will 

 be intermediate. Let us now consider different systems of cell division 



