366 Action of the Genetic Material 



At this point I should Hke to refer to our earHer discussion of 

 biochemical genetics, especially the quotation from Bonner (III 4 C 

 b).To those who are greatly impressed by the apparently simple con- 

 cept of one gene — one synthetic step, such discussions ( Goldschmidt, 

 1920fl, 1927) must appear rather vague and crude. However, when 

 Horowitz found the temperature-dependent nutritional mutants in 

 Neurospora, it became clear that even in those specific and simple 

 cases of genie action, the kinetic element entered, as expressed so 

 strongly in Bonner's words. Thus, biochemical genetics does not lead 

 to a chemical insight into genie action, as was hoped in the beginning, 

 but of necessity returns to the avowedly crude models with which 

 Wright and I approached many problems of genie action. 



The best illustration for the derivation of dominance from a 

 system of different variables in the complex interplay of reaction chains 

 in development comes from temperature studies. It is known that 

 different temperatures affect the phenotype of many mutants in the 

 most varied ways. The action must always take place in a tempera- 

 ture-sensitive or critical phase in development, which is typical for 

 individual loci. Without going into the mass of details (see Gold- 

 schmidt, 1938 ) , the general fact may be understood as an interference 

 with the kinetics of one process in relation to other simultaneous ones. 

 This results in affecting one of the imaginable variables, like potency 

 of the locus, speed of a reaction, threshold for final action, availability 

 and use of a substrate, or other such possibilities, so long as the all- 

 or-none type is not involved, as in the prevention of a synthetic step. 

 The same applies also to changes in dominance which can be produced 

 abruptly by temperature shocks (Goldschmidt, 1935a,Z?; Plough, 

 Child, and Blanc, 1936) and by X-radiation; and which can be ob- 

 tained in an orderly sequence by temperature action in the critical 

 period. 



Only one thoroughly elaborated example studied by the school of 

 Zeleny, especially Hersh ( 1930 ) , may be mentioned. It deals with the 

 dominant position effect Bar in Drosophila (see discussion in I 3 C 

 c aa), the dosage relations of which we have just considered. The 

 number of facets in wild-type eyes and in the mutant series is, ceteris 

 paribus, a function of temperature. If the temperature curves for wild 

 type, mutant, and heterozygote are compared, parallelism would 

 indicate that growth rather than dominance is involved. If the curve 

 for the heterozygotes should not parallel that for the homozygotes, 

 conclusions in regard to dominance might be drawn, since a coefficient 

 for dominance would change over the temperature range. Zeleny had 



