Detection of Induced Lethals 253 



place between the two X chromosomes, and the induced lethal 

 might be transferred from the treated chromosome to the non- 

 treated X chromosome. The CIB chromosomes are used because 

 the crossover suppressor (gene C or an inverted chromosomal 

 segment) prevents such a substitution of a nonlethal for the 

 new lethal mutation in the treated chromosome. The bar gene 

 has no effect but it enables the observer to identify the flies that 

 have the crossover suppressor. Since this is a dominant gene, 

 all females with the treated chromosome and the chromosome 

 with the crossover suppressor can readily be separated from 

 those that have the treated chromosome but have an X chromo- 

 some that lacks the suppressor. 



When the treated CIB females are mated to a normal male, 

 the female offspring are discarded. Half the males fail to sur- 

 vive because they have the old lethal, I. If the radiation treat- 

 ment had induced the appearance of a new lethal, the other half 

 of the males would also die. Examining the offspring easily 

 determines whether males are present. If no lethals were in- 

 duced, the offspring should segregate into 2 females : 1 male; 

 but if a new lethal had arisen as the result of the irradiation 

 the offspring would consist of females only. Gene I is necessary 

 to eliminate the males that lack the treated chromosome. Be- 

 cause of its presence, however, females heterozygous for the CIB 

 chromosome must be used in the original mating because homo- 

 zygous CIB flies do not exist. 



It has been pointed out by Stancati and Whiting that the 

 method of determining the production of dominant lethals by 

 noting a decrease in the number of offspring produced fre- 

 quently fails to distinguish between inactivated sperm and zy- 

 gote lethals. For example, males are irradiated and mated to 

 females, and a decrease in the normal number of offspring re- 

 sults. Is this decrease caused by inactivated sperm, thus pre- 

 venting fertilization or by zygote lethals which have killed the 

 offspring shortly after fertilization? This question is not easy 

 to answer for organisms in which all the individuals must arise 

 from a fertilized egg but can easily be answered in such or- 

 ganisms as Habrobracon juglandis, in which the males with a 

 few exceptions develop from unfertilized eggs. 



If some of the sperm of this wasp are inactivated, the number 

 of fertilized eggs and therefore the number of female offspring 



