400 Action of the Genetic Material 



The scute mutants and position effects remove definite bristles, but 

 different ones in different alleles, which had led to projecting the 

 phenotypic pattern into a chromosomal pattern in Serebrovsky's theory 

 of step-allelism. We do not know whether the unusually large series 

 of multiple alleles controlling the black patterns on the wings of coc- 

 cinellid beetles are also based upon a set of pseudoallelic mutants per- 

 mitting crossing over within their chromosomal section of pigment pat- 

 tern control. But today we should expect such a condition, also for 

 Nabours' closely linked pattern genes in the grouse locust. Whether 

 the same is true for the color spots in cotton we do not know. But for 

 the parallel case of flower and aleurone color in maize (the a and r 

 loci as studied by Laughnan and Stadler) the separation of "two 

 adjacent genes" by crossing over has succeeded. All these are cases 

 in which non-parallelism of pleiotropic effects among multiple alleles 

 had been observed. This suggests that within the chromosomal section 

 of a definite action (e.g., yellow pigment, removal of bristles, deposi- 

 tion of anthocyanin) the details of the effect (its pattern) do not de- 

 pend upon independently controlled local competence but are some- 

 how connected with the location of the disturbance within the pattern 

 of the chromosomal section. (This would be clearly the element of 

 truth contained in Serebrovsky and Dubinin's step-allelism; Muller has 

 repeatedly hinted at his belief that after all there is still something left 

 of this discarded theory. ) At present it is hardly possible to present a 

 convincing interpretation of this situation. But one thing I consider as 

 certain: such facts exclude Lewis' idea of the subsequent biochemical 

 steps controlled by the seriated parts of the chromosomal section, his 

 pseudoallelic genes, since the biochemical effect is always the same, 

 but not the point in space where it takes place. 



c. The time dimension 



In all discussions of the interplay of genie actions the dimension 

 of time is of paramount importance. No orderly development is pos- 

 sible if the individual, genically controlled effects are not properly 

 timed. Thus the timing played a considerable role in our earlier dis- 

 cussions of the meaning of genie activation. We met with it in the 

 discussion of temperature-sensitive periods, of the critical periods for 

 the production of phenocopies, and of the timed series of actions of 

 the multiple alleles at the vestigial locus of Drosophila, and in many 

 other places. The timing of genie effects becomes apparent with 

 special clarity in the action of lethal deficiencies of Drosophila as 

 studied by Poulson (1940, 1945) and Hadorn (1948). Poulson studied 



