Visible Mutations 245 



organisms have also been used and nonlethal as well as lethal 

 mutations have been studied, but the technique which employs 

 the study of lethals in Drosophila is far simpler and the results 

 generally appear to be more reliable. There is no reason to 

 suppose, however, that the simple relationship which was found 

 is unique for Drosophila or for lethal mutations. 



It is customary to measure the intensity of radiation by a 

 unit known as the Roentgen unit or r-unit. Without attempting 

 to explain the nature of X-rays, we can say that an organism 

 can be subjected to the same number of r-units by exposing it 

 to an intense radiation for a short time or by exposing it for a 

 longer period to a radiation of less intensity. In other words, 

 the number of r-units may be varied by varying either the 

 strength of the radiation, the time of exposure, or both. Studies 

 using different amounts of r-units indicate that the relationship 

 between dosage and percentage of induced lethal mutations is 

 probably linear. That is, a certain increase in the amount of 

 r-units will always produce a certain increase in the percentage 

 of lethal mutations. Experimental data show that although this 

 statement is apparently true for low dosages, there is a falling 

 off in the number of mutations detected with higher dosages. 

 This falling off is often interpreted as the result of a defect in 

 the technique used for the detection of lethal mutations, and it 

 is very likely that the linear relationship holds throughout the 

 range of dosages. An interesting feature of these studies is that 

 the extent of lethal mutation will be the same for a certain 

 dosage whether that dosage is the result of a certain intensity 

 of radiation applied continuously or intermittently. 



Visible Mutations 



In general, mutations produced by X-rays or radium are not 

 different from those that occur spontaneously. Many of the 

 mutant types that have appeared spontaneously have reappeared 

 in radiated organisms, and both "visible" and lethal mutations 

 have occurred in radiated and nonradiated material. Reverse 

 mutations have occurred after treatment with X-rays or radium 

 and are similar to reverse mutations that have occurred spon- 

 taneously. The chief difference is that under radiation, many 

 more mutations will take place in a given unit of time than 

 under normal conditions. For example, in the X chromosome of 



