Mutagenic Specificity and Directed Mutation 1 



HAROLD H. SMITH 



Brookliaven National Laboratory, 

 Upton, L. 1., N. Y. 



Directed control of induced mutation is an important objective 

 in both theoretical genetics and its practical application. The 

 means of achieving and improving this control is through better 

 understanding of mutagenic specificity. Evidence of specificity has 

 been reported for mutations among genes (inter-locus) and among 

 alleles (inter-allelic and/or intra-locus), for localization of chromo- 

 somal breakpoints, and for relative frequencies of gene mutations vs. 

 chromosome breaks. 



In the first part of this paper, experimental evidence will be sum- 

 marized for a nonrandomness, differential effect or specificity of 

 induced mutation in organisms ranging from higher plants and 

 animals to bacteriophage. The selectivity may be expressed in broad 

 spectrum experiments of "forward" mutations and chromosome 

 breakage, or, more specifically, in differences of back mutation 

 response of particular alleles to a series of mutagenic agents. This 

 discussion will be followed by considerations relating interpreta- 

 tion of mutagenic specificity to mechanisms of action and structure of 

 the genetic material. 



Evidence for Mutagenic Specificity 



Higher Plants 



The evidence for mutagenic specificity in higher plants is con- 

 fined for the most part to "forward" (impaired function) mutations 

 between loci (inter-locus specificity) and gene mutations vs. chromo- 

 some breaks. The Swedish group of investigators have been most 

 active in this field and have reported results on different spectra of 

 mutations produced in barley following treatment with different 

 mutagens (28, 29, 71). 2 Chlorophyll mutations of the nitidis type are 



Research carried out at Brookhaven National Laboratory under the auspices of the 

 U. S. Atomic Energy Commission. 

 = See References, page 430. 



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