308 1'IIYSIOLOGICAL GENETICS 



and other authors have confirmed this result. In general terms, 

 we are dealing with the same type of position effect at or near a 

 break, but there is the additional fact that the Plum effect appears 

 only when the material from the brown locus comes in touch 

 with the chromocenter. 



5. In a further study of the last-mentioned Plum rearrange- 

 ments, Dubinin (1936) found what he considers to be a new type 

 of position effect. By further breaks in the inverted region of 

 the second chromosome, which is responsible for the Plum effect 

 by bringing a section of inert material in contact with the broken 

 chromosome, he produced chromosomes in which both ends of the 

 inversion are in contact with inert material. He found that 

 considerable regions on both ends of the inversion produce the 

 Plum effect if contiguous with inert material. The position 

 effect is therefore not a point effect in this case but an effect of 

 different regions. This result ought to be considered in connec- 

 tion with the frequently found position effect not at but near the 

 break. 



B. Interpretation in Terms of Genes 



Different interpretations have been proposed to explain such 

 facts without changing the concept of the gene. We mention 

 only the idea that in all these cases a mutation arises near or 

 at the locus of breakage, which has been disproved in some of the 

 instances reported; or the idea that small deficiencies are caused 

 at the points of breakage, which also does not agree with some 

 of the facts. The position effect as such has therefore to be 

 accepted, and the problem is, to determine what it actually 

 means. Dobzhansky (1936) expresses himself in a more general 

 way: 



The activities of a gene lying in an abnormal position in the chromo- 

 some are deflected from their normal course by influences emanating 

 from its new environment but no permanent alteration is wrought in the 

 gene itself. . . . Position effects may prove to be essentially develop- 

 mental phenomena, but phenomena of intracellular physiology, affecting 

 the coordinates on which the reactions between the primary gene prod- 

 ucts take place. . . . The hereditary material is discontinuous, for it 

 is segregated into independent units, genes. And yet, it is a continuum 

 of a higher order; since the independence of the units is incomplete, they 

 are changed if their position in the system is altered. 



