146 Applied Biophysics 



determined satisfactorily only for the group of recessive sex- 

 linked lethals, though sufficient has been done with visible reces- 

 sive mutations and with mutation in other organisms to suggest 

 that the results are characteristic. First of all, however, it should 

 be mentioned that the natural mutation rate in Drosophila 

 melanogaster (measured by sex-linked lethals) increases with 

 the age of the tissue tested and with the temperature at which 

 it is kept. Further, it differs from stock to stock and in a few 

 cases may be relatively high. Thus, Demerec ^' found that the 

 Florida stock gave about 1% of sex-linked lethals, the average 

 of all other stocks being about 0.1%. This he found to be due 

 to a recessive gene, located on the second chromosome, which 

 raised the general mutation rate of all the genes in the organism. 

 This behavior is to be contrasted with the case found by 

 Rhoades -^ in maize, where the gene Dt increases the mutation 

 rate only of the gene ai. 



The mutation rate induced by X-rays is found to be linearly 

 proportional to the dosage. The frequency of sex-linked lethals 

 induced in Drosophila sperm is about 3% per 1,000 r.-^ This 

 rate is independent of the wave length of the radiation through- 

 out the gamma ray and X-ray range up to a wave length of 

 2.6 A. It is independent of the time occupied by the irradiation, 

 i.e., is independent of intensity down to the lowest tested (0.07 r 

 per minute) and of whether the dose is fractionated or given in 

 one exposure. Lastly, it is unaffected by temperature and is 

 probably independent of the natural mutation rate of the par- 

 ticular stock employed. Timofeeff-Ressovsky ^^ should be con- 

 sulted for full details. 



These facts indicate that the induced mutations must be due 

 in quite a direct manner to a single ionization excited in a sensi- 

 tive volume which may be the gene itself or include the gene 

 or some part of it.'^^ The ionization adds considerable energy 

 to the affected gene, and the excited molecule, rendered tem- 

 porarily unstable, is enabled to slip from one relatively stable 

 chemical state to another. What the precise change may be is 

 imknown, but any change in the gene molecule may be expected 

 to alter the properties of the whole gene and so to be disclosed 



