202 CHAPTER 23 



cation is resumed. Finally, the possibility new gene is completed and attached to its 

 still remains that changes can occur in the linear neighbors; these changes may be de- 

 steps leading to gene synthesis, before the tected later as point mutants. 



SUMMARY AND CONCLUSIONS 



The mutational units in a genotype are, in order of size, the genome, fhe chromosome, 

 chromosomal segments involving more than one gene, and the gene. Since a number of 

 different alleles occur per gene, gene mutation may involve the entire gene, or one mutational 

 site within the gene which has many alternatives, or many mutational sites which have one 

 or more alternatives. It is possible that the genes operationally delimited by recombination 

 and by mutation are not exactly equivalent materially. However, in the absence of critical 

 evidence on this, we shall continue to assume that they are identical. 



Gene mutation is not limited in any way with regard to the effect it can have on synapsis. 

 It is also in no way restricted by ploidy, type of cell or gene, but is limited with respect to 

 the effect it can have on a gene's polarity. Tripolar genes are excluded, bipolarity being the 

 usual, and unipolarity the less usual, alternative. 



Point mutations are the remainder of all mutations which are not known to involve inter- 

 genic changes. Since point mutations include changes in single genes they may be em- 

 ployed to determine the mutational characteristics of the gene. The frequency of point 

 mutations increases linearly with the dose of high energy radiations, there being no effect 

 of dose protraction, and no threshold dose below which the gene is safe from such change. 

 Such mutations indicate that a given gene is relatively stable over many cell generations, 

 changes in it being the result of very localized physico-chemical events which are completed 

 in a matter of minutes, after which the new gene is similarly stable. Changes in the gene are 

 enhanced or induced by temperature changes, aging, gene replication, and physical and 

 chemical mutagens. It is possible that changes resulting in gene mutation can take place 

 in the old gene, or in the new gene, or during the formation of the new gene. 



REFERENCES 



Alexander, P., "Radiation-Imitating Chemicals," Scient. Amer., 202, No. 1 :99-108, 1960- 



Muller, H. J., "Variation Due to Change in the Individual Gene," Amer. Nat., 56:32-50, 

 1922. Reprinted in Classic Papers in Genetics, Peters, J. A. (Ed.), Englewood 

 CliflFs, N.J., Prentice-Hall, 1959, pp. 104-116. 



Muller, H. J., "Artificial Transmutation of the Gene," Science, 66:84-87, 1927. Reprinted 

 in Classic Papers in Genetics, Peters, J. A. (Ed.), Englewood Cliffs, N.J., Prentice-Hall, 

 1959, pp. 149-155, and also in Great Experiments in Biology, Gabriel, M. L., and 

 S. Fogel (Eds.), Englewood Cliffs, N.J., Prentice-Hall, 1955, pp. 260-266. 



QUESTIONS FOR DISCUSSION 



23.1 . Is there a dose of X rays and/or of ultraviolet radiation which is safe, in that it cannot 

 cause some point mutations? Explain. 



23.2. Can we be sure that any given mutation involves a single gene change rather than an 

 intergenic one? Explain. 



23.3. Would we know of the existence of genes if all genes had the identical mutational 

 capacity? Explain. 



