Discussion of Session I 



M. M. RHOADES 



Indiana University, Bloomington, Ind. 



At a symposium concerned with mutation and plant breeding it 

 l would seem appropriate to consider briefly just what is meant 

 by the term mutation and to describe some of the inherited changes 

 in chromosomal organization which lead to modifications of the 

 normal phenotype and hence are called mutations. 



Many hold that mutations, either of spontaneous or induced 

 origin, stem in large part from some kind of intragenic change at the 

 molecular level, i.e., they are true or intragenic mutations. Inasmuch 

 as the genetic information is determined by the combinations of the 

 two pyrimidines and the two purines of DNA, it is believed that the 

 substitution of one purine or of one pyrimidine for another, or of a 

 purine for a pyrimidine and vice versa, would produce a new kind of 

 genetic code which would lead in some cases to an altered phenotype. 

 A change in gene action producing a mutant phenotype could also 

 conceivably occur by an inversion in base order or by a deletion or 

 duplication of one or more base pairs. That mutations of these kinds 

 do in fact occur appears highly likely from the studies by Freese and 

 Benzer on the mutagenic effect of base analogues and other chemical 

 mutagens in phage. Some mutagens are thought to induce mutations 

 in duplicating DNA and others in non-duplicating DNA. Comparable 

 though less extensive results have been obtained with bacteria. 

 Muller, Carlson, and Schalet suggest that a rotational substitution, 

 whereby the two organic bases of a nucleotide pair become freed from 

 the backbone and reversed in position upon reattachment, may be 

 responsible for the whole body mutations in the non-replicating DNA 

 of Drosophila sperm. 



Granting that intragenic mutations occur in phage and bacteria 

 there is virtually no convincing evidence for intragenic changes in 

 mutation studies of higher forms. This does not mean that intragenic 

 changes do not occur in these organisms, and indeed on theoretical 

 grounds it is difficult to deny that they do arise, but only that there are 

 a number of extragenic events which simulate gene mutation. In prac- 

 tice those mutations which cannot be ascribed to one or another 

 extragenic mechanism are tentatively placed in the category of gene 



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