222 



CHAPTER 25 



fairly common in maize, and the phenotypic 

 instability of various loci in other flowering 

 plants, ferns, fungi, and bacteria may be 

 due to similar factors. A genetic control of 

 spontaneous "breakability" of chromosomes 

 that resembles the Ac-Ds system has been 

 found also in Drosophila melanogaster. We 

 do not know to what extent transmissible 

 changes are due, not to point mutations of 

 the "mutable" and "less mutable" loci, 

 whose phenotypic eff"ects we are following, 

 but to the removal of some neighboring 

 gene whose presence can cause a position ef- 

 fect. However, not all point mutations can 

 be such position eff'ects, of course, since dif- 

 ferent genes must first arise by mutation in 

 order to obtain position effects with them. 



Royal Alexander Brink in 1961. 



SUMMARY AND CONCLUSIONS 



The spontaneous occurrence of genomic and of single whole chromosome mutations is sup- 

 pressed by the genotypic control of the processes of mitosis and meiosis. Structural rear- 

 rangements in chromosomes are suppressed by the precision of synapsis. All these controls 

 are possible only because genes are linearly arranged in chromosomes with sealed-off ends, 

 such structural features themselves being primary properties of genes. Genetic change is 

 genotypically regulated in certain cases involving the production of polyploid and polytene 

 chromosomes, and of several monocentric chromosomes from a polycentric chromosome. 



Point mutation frequencies also are regulated genotypically. This is evidenced by the 

 general control of mutation response to temperature changes or to mutagenic agents, by 

 the occurrence of mutator genes, and of genes which produce breakages in chromosomes 

 that lead to losses, shifts, and transpositions which may cause position effects, and by the 

 occurrence of genes which regulate the operation of the genes causing the breakages. 



REFERENCES 



Brink, R. A., "Very Light Variegated Pericarp in Maize," Genetics, 39:724-740, 1954. 



McClintock, B., "The Origin and Behavior of Mutable Loci in Maize," Proc. Nat. Acad. 

 Sci., U.S., 36:344-355, 1950. Reprinted in Classic Papers in Genetics, Peters, J. A. 

 (Ed.), Englewood Cliffs, N.J., Prentice-Hall, 1959, pp. 199-209. 



McClintock, B., "Controlling Elements and the Gene," Cold Spr. Harb. Symp. Quant. 

 Biol., 21:197-216, 1956. 



Peterson, P. A., "The Pale Green Mutable System in Maize," Genetics, 45:115-133, 1960. 



