THE SUMMING UP 389 



apparently different classes of position effect were described: tfie S-type 

 and the V-type. Tfie S-type, in whicfi two pseudo-allelic mutations 

 produce only mutant pfienotypes when located on separate chromosomes 

 {trans), but the wild-type phenotype, when their -j--alleies are recom- 

 bined on the same chromosome (cis), is now called the cis-trans position 

 effect. This phenomenon, as we have seen in Chapter 6, is now inter- 

 preted as the requirement for continuity of a functional unit consisting of 

 hundreds of nucleotides. 



The V-type position effect, however, originally recognized as the in- 

 fluence of a neighboring region upon the expression of a particular 

 gene is not understood at all. It is only now becoming possible to 

 reopen the investigation of this phenomenon in which both chromosomal 

 structure and gene function are intimately involved. In fact, the 

 dichotomy between structure and function, so useful in the early devel- 

 opment of genetics, is now being replaced by a more integrated ap- 

 proach, essential for a more penetrating analysis of genetic mechanisms. 

 A main limitation in the development of an integrated view has been in 

 the area of gene action. 



GENE ACTION 



No over-all picture of gene action can be synthesized from the existing 

 evidence with any sense of assurance. Nonetheless, it may be worth- 

 while to reorganize the material presented in Chapters 9, 10, and 11 

 in a more unified form. We shall attempt to do this by considering the 

 various elements of a working genetic system which have been described, 

 together with some proposals about their modes of interaction. 



At the chromosome level, three kinds of determinants have been 

 described. They are the functional genes, which we shall refer to 

 collectively as S, the controlling elements, O, located adjacent to S, 

 active only in the cis position, and able to repress the expression of wild- 

 type A, and another set of controlling elements, R, which mav be located 

 anywhere in the genome, and which act by activating O. In addition to 

 these elements, there may also be nonchromosomal genetic factors in- 

 volved in the action of every gene. All these determinants have been 

 identified operationally by their phenotypic effects. How do they cor- 

 respond to chemically defined components? 



It seems a reasonable extrapolation from known data that the chromo- 

 somal components are DNA, but even here there is some doubt; perhaps 

 some are non-DNA constituents. The difficulty in detecting a regularly 

 associated protein in the bacterial chromosome tends to exclude protein 



