MANNER OF PRODUCTION OF MUTATIONS 525 



some influence of this kind. However, if these longer range effects do 

 exist in Drosophila spermatozoa and Tradescantia pollen treated with 

 ionizing radiation, they must be very rare in comparison with the ultra- 

 microscopically circumscribed effects, otherwise the results from neutrons 

 would not have shown as much localization of pattern as they do. 



9. THE TARGET HYPOTHESIS AS APPLIED TO MUTAGENESIS 



The finding that the hits of ionizing radiation, in order to be muta- 

 genically effective, must usually occur in the immediate submicroscopic 

 vicinity of the genetic material which becomes altered thereby does not 

 constitute proof that these hits must actually fall inside or upon the given 

 gene or in any fixed region within or associated with the gene, or even 

 inside the chromosome. If, however, this primary assumption were true 

 even approximately, as the "target hypothesis" supposes, and if it were 

 also true (2) that each such ionization gave rise to mutation in the given 

 gene, (3) that virtually no excitation did so, and (4) that practically all 

 these mutations were detectable, then it would be possible to use existing 

 data on the efficacy of radiation mutagenesis for calculating the size of 

 the gene or at least its sensitive volume, and the number and to some ex- 

 tent even the shape of these units. This has repeatedly been attempted. 



The target hypothesis was first applied to biological material by 

 Crowther (1924, 1926), in an attempt to estimate the size of the body 

 controlling whether or not mitosis shall occur in an irradiated cell, on the 

 tentative postulates (corresponding with those given above) that any 

 ionization but no ordinary activation within this body and no ionization 

 outside it would distinctly inhibit mitosis. The volume thus calculated 

 was found to correspond fairly closely with the apparent size of the 

 centriole as visible in stained preparations. The hypothesis was again 

 apphed by Blackwood (1931, 1932) for similarly estimating the size of the 

 gene, or, to use his qualification, of its "sensitive region," on the corre- 

 sponding postulates that any ionization but no mere excitation within 

 this region of a given gene and none outside it would give rise to a detect- 

 able mutation of it. 



The essential principle involved in this method of calculation is simple 

 and may be expressed as follows. Suppose that i represents the chance 

 (exceedingly small) of ionization of any individual atom, selected at 

 random from the sensitive region of a given gene, when a given dose is 

 applied, and that a is the number of atoms within that region. Then, if 

 the ionizations are randomly distributed the product at must on this 

 hypothesis be equal to the chance m of a mutation being produced in that 

 gene by that dose. For the purpose of determining the number of atoms 

 in the gene, the equation m = ia may be rewritten a = m/i. Now i, the 

 chance or frequency, at a given dose, of ionization in material having the 



