166 Nature of the Genetic Material 



when stretched. It is now assumed that the synaptic association of 

 chromosomes in Diptera leads to a stretching of chromosomal molecu- 

 lar structure. The presence of a rearrangement may lead, through 

 changes in the pairing condition, to modifications in the state of 

 stress in the immediate vicinity of breakage points. 



I do not think that a more detailed discussion of this hypothesis 

 is needed. The paired condition does not exist outside the group of 

 the Diptera, but position eflFect does. The group of facts presented 

 in the foregoing pages lends no support to the hypothesis and does 

 not derive any explanation from it. The former discussion shows that 

 it is not possible to take position effect as a kind of freakish and 

 specialized feature which has to be explained away rather than under- 

 stood. It shows that position effect, mutation, chromosomal structure, 

 and the theory of the gene are closely intertwined aspects of the 

 nature of genie material and therefore must be explained as a whole. 

 Thus I do not think that this theory helps us to understand the facts 

 as they are known now. However, this type of theory, if separated 

 from the no longer tenable classic theory of the gene, is of the same 

 general structure required for our discussion here: it stresses what 

 may be called field actions within the chromosome, in preference to 

 chemical changes within a supposed gene molecule. 



5. A kind of compromise between the genie and non-genic 

 theories of position effect has been proposed by Serra ( 1949 ) . We 

 have mentioned previously that he considers "heterochromatization" 

 of euchromatin a fact, meaning that heterochromatin is matrix nucleo- 

 protein which may envelop euchromatin and impede its functions, 

 thus leading to the mottling effect. The general idea has been criticized 

 before (I 2 C c). It was pointed out that such views do not take 

 care of the non-mottled position effect. We have noted also that 

 Lewis ( 1950 ) considers the two types of effects ( his S and V effects ) 

 as completely different, a point of view which we refuted. Now 

 Serra introduces a completely different explanation for the standard 

 (S) position effect: it is due to the genes being composite and 

 becoming separated into two parts, which can no longer work 

 together as a + allele. This means that a gene which is a composite 

 of subunits breaks up and its parts are relocated at different points, 

 causing inactivation. Rearrangements cause a sequence of genetic 

 materials capable of forming a new gene. This has no equivalent 

 + allele and now can mutate; that is, a position effect results, which 

 is no more than another name for an incipient new gene. 



Following up these ideas to encompass mutation also, it is 



