124 MUTATION AND PLANT BREEDING 



suffers from the fact that the dose of a chemical as applied exter- 

 nally bears only a remote relation to the amount that reaches the 

 genes. We used a more indirect approach in experiments on Droso- 

 phila (68). The effective dose of mustard gas was measured in terms 

 of sex-linked lethals, and against this was plotted the frequency of 

 translocations in the same tests. We found that translocation fre- 

 quency increased almost exactly as the square of lethal frequency, 

 and we took this to mean that mustard gas, like X-rays, acts by inde- 

 pendent "hits" in or near the genetic material, one hit being suffi- 

 cient to produce a lethal while two hits are required for a 

 translocation. 



Corroborative evidence came from experiments in which treat- 

 ment of Drosophila S $ by mustard gas was immediately preceded or 

 followed by X-rays (73). With both arrangements, the frequency 

 of lethals after the combined treatment was the sum of the fre- 

 quencies produced by either treatment separately. That of trans- 

 locations was higher and calculation showed that it agreed well 

 with what would be expected if rearrangements were formed indis- 

 criminately from broken ends whether these had been produced by 

 X-rays or mustard gas. It is interesting to note that Wolff and Luip- 

 pold (93) obtained a different result when they exposed plant chro- 

 mosomes to a combined treatment with neutrons and X-rays. In 

 these experiments the frequency of interchanges was the sum of the 

 frequencies that had been produced separately by either radiation. 

 This was attributed to the fact that breaks produced by the closely 

 ionized neutron track are not scattered at random throughout the 

 nucleus, but lie close together and therefore tend to rejoin with 

 each other rather than with independently produced X-ray breaks. 

 It may be concluded that mustard gas, unlike neutrons, produces 

 breaks which are scattered randomly. 



Urethane 



The ability of methane to produce translocations in flower- 

 ing plants was discovered by Oehlkers (71) at the beginning of the last 

 war. Subsequently, the mutagenic action of urethane was con- 

 firmed for Drosophila (92). Neurospora, on the other hand, has 

 shown itself wholly refractory to its action (46). This may be con- 

 nected with the fact that urethane shows an unusual degree of 

 organism specificity also in its carcinogenic action. Rogers (77) found 



