98 Applied Biophysics 



state of nutrition, and degree of germination, affect the muta- 

 tion rate.^^ Most gene mutations are recessive, i.e., able to pro- 

 duce their characteristic effect only when paired with another 

 mutated gene of the same kind. Only a minority produce any 

 conspicuous morphological abnormality. Occasionally a change 

 in the gene occurs which initiates new developmental processes,'**^ 

 A mutation caused by one irradiation may be reversed by a subse- 

 quent exposure. ^'^^ This is exceptional, however, and in nearly 

 every case the mutation effect is exactly proportional to the 

 amount of energy received, and exactly cumulative over an 

 indefinitely long period even in successive generations. It is 

 unknown to what extent these observations are applicable to 

 man. 



Thus, radiation can be regarded as a useful tool in purely 

 genetic investigations on such problems as the properties of 

 genes and chromosomes, the size and number of genes and their 

 mutational potentialities. Investigations on the genetic effects 

 of radiations provide valuable data on one of the ways in which 

 biological material responds to radiation, but, as rightly empha- 

 sized by one of the foremost genetical investigators, "Not all 

 the effects of radiation in killing organisms or disturbing their 

 development are referable to changes either of the class of gene- 

 mutations or chromosome re-arrangements." ^^ 



Injurious and Lethal Effects of Radiation 



In previous sections some account has been given of the in- 

 juries caused to small organisms (biological indicators) and to 

 particular organs within cells (chromosome effects) by penetrat- 

 ing radiations. There still remains to be considered the largest 

 field of inquiry within the domain of experimental radiology, 

 namely, studies of the effects of radiation upon complex tissues 

 both in health and disease and after experimental injury. 



Innumerable observations have been made of the effects of 

 radiation, under the greatest variety of physical conditions, upon 

 embryological development, the various systems of the body at 

 different stages of growth, individual organs and on the body 



