496 RADIATION BIOLOGY 



Bridges' map, a random distribution of distances between breaks, when, 

 as in the other study, allowance is made for the lower viability of the 

 larger deletions. At the same time, extremely minute deletions, of less 

 than four bands, although infrequent relatively to the total, occur much 

 oftener than they would on a random distribution of breaks and unions. 

 Although these results do not in themselves prove the conclusion that 

 deletions of more than three bands usually involve two independent 

 breaks, certain other recent studies (J. I. Valencia, 1952, and unpublished 

 data) indicate that euchromatic deletions of such a size as to be readily 

 visible in the salivary-gland chromosomes (i.e., those of more than two or 

 three bands) do not arise at low doses at as high a frequency as would be 

 expected if the rule of linear proportionality to dose held true for any 

 considerable portion of them. These studies are not yet completed or 

 definitive, and the matter must therefore be regarded as requiring further 

 clarification. But, however it turns out, it will (as indicated in Sect. 3) 

 have a bearing on the interpretation of the observed linear relation 

 between the frequency of induced lethals in general and the dose of 

 radiation if, as has often been supposed, a high proportion of lethals 

 induced by ionizing radiation consists of small deletions. 



5. POSSIBLE RELATIONS BETWEEN POINT MUTATIONS AND BREAKS 



PRODUCED BY RADIATION 



It is a noteworthy fact that both ionizing radiation and ultraviolet, 

 and in fact all mutagenic agents concerning which good evidence on the 

 matter has been obtained, including even ''mutator" genes, have been 

 shown to give rise to both point mutations and chromosome breaks, even 

 though they may do so with different relative frequencies. This general 

 parallelism suggests a similarity in the nature of the physicochemical 

 bonds between genes and those within (i.e., between the parts of) genes. 

 That these bonds do, nevertheless, differ from one another in some sys- 

 tematic way has been indicated by the evidence (see Chap. 7) for the 

 conclusion that the genes form discrete units, resulting in a segmental 

 arrangement of the genetic material. However, regardless of whether or 

 not this is true, the above qualitative parallelism raises the question, 

 brought up independently by a number of authors, can it be that point 

 mutations and breaks are individually associated, in the sense that they 

 more or less regularly arise in close proximity with one another? If this 

 is so, is the point mutation the cause of the break or the consequence of it, 

 or are both phenomena separate consequences of an antecedent disturb- 

 ance produced by the radiation? 



Undoubtedly small deletions are consequences of chromosome break- 

 age, and they come under the empirical category of point mutations. 

 The more important question, however, is concerned with those changes 



