Detection of Induced Lethals 251 



of a band or of a few bands in the salivary gland chromosomes 

 even though ordinary somatic chromosomes would not reveal 

 such a small loss of chromatin material. Many lethals, how- 

 ever, cannot be detected cytologically even in such favorable 

 material as the giant chromosomes of the salivary glands. The 

 question then arises whether these lethals are true gene muta- 

 tions or whether they are chromosomal deficiencies that are too 

 small to be detected by present cytological methods. 



Probably some lethals are true gene mutations. Gene muta- 

 tions may be induced which produce a visible effect; by analogy, 

 some of the lethals for which there is no known cytological 

 cause may be actual gene mutations. An attempt to correlate 

 the number of induced dominant lethals with the number of 

 induced chromosome aberrations shows that a dosage of 1000 

 roentgens will produce a much larger percentage of lethals than 

 of cytologically detectable changes in the chromosomes. That 

 all lethal mutations are not structural changes is indicated by 

 the temperature studies of Plough and others who found that the 

 mutation rate of fruit flies could be increased by raising the 

 larvae at temperatures higher than normal. Sometimes lethal 

 mutations revert to the nonlethal condition, indicating that 

 these lethals are gene mutations, for it is difficult to visualize 

 how a deficient segment of a chromosome can reappear. 



Detection of Induced Lethals 



Much of our information on the frequency with which lethal 

 mutations are induced by radiation comes from studies on Dro- 

 sophila melanog aster. For detecting the presence of a new lethal 

 in the X chromosome of that fruit fly, a very clever technique 

 has been devised which is very simple to use and by which re- 

 sults may be obtained in large numbers. It is known as the CIB 

 method (Fig. 76). 



To estimate the frequency with which induced lethal muta- 

 tions are produced in the X chromosome, male flies are exposed 

 to radiation of known kind and intensity and are then mated 

 with nonradiated females. If a lethal is induced in the X chro- 

 mosome of the male it will not be detected in the female off- 

 spring because it will be "covered up" by the nonlethal in the 

 X chromosome received from the female parent; but when these 

 female offspring are subsequently mated to normal males, the 



