426 MUTATION AND PLANT BREEDING 



tion, chemical composition, and possibly in the organization of genet- 

 ic fine structure. In higher plants and animals there is now evidence 

 that the chromosome consists in cross section of 32 DNA strands 

 before synthesis and 64 after DNA synthesis in prophase through 

 metaphase (63). In contrast, from T2 phage to bacteria the number 

 of DNA strands is considered to be usually 2, with exceptions. Fur- 

 thermore, in phage, and apparently in bacteria, there is no close 

 association of protein with the DNA as in the nucleoprotein composi- 

 tion of chromosomes in higher forms. It is not yet clear that gene loci 

 of higher forms are subdivisible into units of mutation, recombina- 

 tion, and function in the same way as complex loci in microorganisms; 

 but the presence of pseudo-alleles in Drosophila and maize suggests 

 that a basic similarity of organization may exist. Westergaard (70) has 

 discussed further differences between genetic mechanisms in micro- 

 organisms and higher forms which may be significant in specificity. 



The evidence for mutagenic specificity in higher plants and ani- 

 mals rests primarily on (a) the different spectra of gene mutants pro- 

 duced by different mutagens in barley and in Drosophila; and (b) 

 the differential effect of certain mutagens on chromosome breakage 

 both with respect to localization of breaks (usually in heterochromatic 

 regions) and in the ratio of gene mutations to chromosomal aberra- 

 tions. A common cause for many of these results may be that there is 

 regional (particularly heterochromatic) sensitivity of the chromo- 

 some to damage by different mutagens and that the expressed change 

 may vary in degree with the mutagen to give gross chromosomal 

 breaks, small deficiencies (lethals), or "visible" gene mutations. (For 

 more detailed discussion, see 4, 5, 17). It lias been suggested by West- 

 ergaard that the higher proportion of visibles over .lethals in Droso- 

 phila and the higher proportion of less drastic viridis over more 

 extreme albina chlorophyll mutations in barley may reflect in each 

 case a less drastic effect in the mutation event of certain chemical 

 mutagens compared to ionizing radiations. 



A large gap remains to be bridged between the intra-locus or site 

 specificity observed in microorganisms and the inter-locus or chromo- 

 somal region specificity observed in higher plants and animals. The 

 former implies a correlation between hereditary function and struc- 

 ture within DNA molecules; the latter may depend more on the chem- 

 ical bonds that hold the DNA molecules together and also conceiv- 



