GENETIC CONTROL OF CELL INTEGRATION 345 



3. \4cClintock has described two kinds of controlling elements: those 

 like Ds which influence only the locus at which they are inserted (they 

 must be in the cis position), and require the presence of another con- 

 trolling element elsewhere in the genome to activate them; and those like 

 Ac which can act either on the spot, or at a distance, provided a suit- 

 ably receptive site exists within the locus which is affected. 



Both kinds of controlling elements can elicit a tremendous diversity 

 of mutationlike responses. The reversibility of the phenotypes demon- 

 strates that position effect is a more accurate description than mutation. 

 Nonetheless, in the Ds-Ac system, if a locus is repressed by the pres- 

 ence of Ds, removal of Ac from the organism by outcrossing results in 

 locking Ds into place, making its effect an irreversible one. Thus, muta- 

 tions in the sense of changes in nucleic acids at the chemical level may 

 be operationally indistinguishable from position effects. 



McClintock s material was probably exceptional because of all the 

 chromosome breakage involved in its origin. It must be borne in mind, 

 however, that most of her observations have now been confirmed with 

 other strains of maize, many of known pedigree. It is her view that the 

 controlling elements which have gone out of kilter and, therefore, re- 

 vealed themselves in her material, play a central role in normal metab- 

 olism and development. The recent findings of gene-controlled repres- 

 sion, in some cases apparently operating at the chromosome level (o 

 genes and X-gal induction in E. coli) support the view that position effects 

 may be of great significance in wild-type systems. Presumably the posi- 

 tion effects would not normally be exerted by transpositions or by phages, 

 but rather by the action of cell metabolites upon susceptible sites which 

 are permanently located within loci, and capable of exerting local position 

 effect repression on neighboring gene sites. 



4. Further insight into the remarkable properties of genes revealed in 

 maize comes from the demonstration by McClintock that there are dif- 

 ferent systems of controlling elements, with particular alleles respond- 

 ing exclusively to one controlling element and not to others. As an 

 example, let us consider the a^ locus in chromosome 3, which influences 

 pigment formation in seed and plant, and is epistatic to a number of 

 other loci scattered through the genome. 



Instability at the a^ locus was first studied by M. M. Rhoades, 

 who showed that it results from the presence of a gene called Dotted 

 {Dt) in chromosome 9. When Dt is crossed into plants of the genotype 

 flj dt, the a J locus loses its stability. A high frequency of somatic 

 and germinal mutations occurs, giving rise to kernels with clones of pig- 

 mented cells (i.e., dots) on a colorless background (see Fig. 2.8), and to 

 pigmented streaks on stems and leaves. The germinal mutations which 



