152 Nature of the Genetic Material 



further because a proper understanding of the similarities and dif- 

 ferences between the variegation effects in DrosopJiila and maize is 

 essential for a derivation of general conclusions upon the theory of 

 the gene. We shall deal with some of the former statements in dif- 

 ferent language, and enlarge upon them. In Drosojjhila — assume eye 

 color as a model for all other cases — variegation is the result of a 

 rearrangement break which brings block heterochromatin near an 

 unchanged normal locus. The mutant type (e.g., white eyes) is the 

 product of a position effect, while the mottled type results from a 

 weakening of the effect in the presence of heterochromatin together 

 with a threshold condition for the position effect ( white ) , which is not 

 passed in all primary pigment cells, because of simple statistical vari- 

 ation of the action among many similar cells. 



It is instructive to see how this effect in Drosophila would have 

 to be described in the terminology of the work on maize. White eye 

 would then be a simple mutant w, which became mutable toward 

 normal W in some cells in the presence of a mutator like Ds in maize, 

 known to be connected with rearrangements. However, this mutator, 

 the rearrangement, would not act except in the presence of another 

 dominant condition (the Ac in maize), which in Drosophila is, of 

 course, the condition that heterochromatin has been transposed near 

 the position effect locus. It is obvious that the maize variegation with 

 all its peculiarities should be understandable in terms of the Dro- 

 sophila effect, as the position effect of a break upon a known locus and 

 the weakening of it, in a statistical way, around a threshold by some 

 property of the rearrangement, which does not necessarily have to do 

 with heterochromatin. To this is added the variation of the effect 

 under the influence of other conditions, the type of rearrangement, 

 and all the things described as states in McClintock's terminology. If 

 this conclusion applies to all so-called mutable loci, we could answer 

 our former question of whether or not these are "sick genes," or some 

 such specific conditions of a "gene," by saying (1) there are no 

 mutable loci; (2) the loci responsible for the variegation effect are 

 unchanged normal loci; (3) these loci are located in the neighborhood 

 of a rearrangement break causing a position effect; (4) this effect is 

 variable because of some weakening effect of the heterochromatin or 

 other unknown features. The effect in maize is not necessarily the 

 same as in Drosophila: it does not require the presence of hetero- 

 chromatin adjacent to the break. This variability includes change in 

 both directions: more variegation up to normality, or less down to the 

 ordinary mutant type. 



