1212 



BIOLOGICAL EFFECTS OF RADIATION 



Discarded 



Tested 



stock with convenient marking genes afford the method. By the proper 

 further crosses, it is then possible to ascertain the presence and locus of the 



lethal. 



This procedure has 

 certain drawbacks. 

 Counts must be suffi- 

 cient to make the 

 departure from the 

 normal ratio signifi- 

 cant. The viability of 

 the marking genes 

 must be adequate to 

 avoid errors resulting 

 from fluctuations due 



Fig. 1. — The CI B technique for the measurement of to the marking genes 

 lethal mutation rate in the A'-chromosome of Drosophila 4.u„„-,„„i.,„(j T4- wac 

 mdanogaster. This method is especially adapted for the l^nemseives. 11 was 

 detection of the effects of environmental agents on the male, therefore advisable, 

 since the tested X-chromosome is paternal. The lethal in , r.ricciK1o fr.r Mullor 



the CI B chromosome kills all males which carry it; the ^"^ possiDie lOr iViUUer 

 other X-chromosomes are generally distinguished by the use in DrosophUa, to utilize 

 of marking genes, here denoted by the letters a, 6, c, d, e. refinements of tech- 

 nique which made the departures from a normal ratio more striking. 



In Drosophila, and wherever the male is the heterozygous sex, a new 

 sex-linked lethal reveals its presence by a ratio of two females to one 



male. The use of sex- 

 linked marking genes 

 provides a further 

 check ; the surviving 

 males must bear a 

 special relation to the 

 marking genes, depend- 

 ent on the locus of 

 the lethal. Now if by 

 some means, crossing 



Cy 



$ 



X 



Cy 



X 



cT Treorted 



iL 



Cy 



? 



X 



Cy 



I? 



d 



c/ 



over is suppressed 

 between the chromo- 



1 All surviving offspring are Cy if <x lethal is present 



Fig. 2. — An experiment for the detection of lethals in 

 L3«%TviQo Jn rinQotiz-vn ntiKr the sccond chromosome of Drosophila melajiogaster . The Cy 



nUiUcb 111 UUcoUHJll, Ulliy , !• • . ,1 • iL f 1 1 



chromosome eliminates the cross-overs in the lemale, ana 



one type of male usually the homozygote does not survive. Either males or 



should aDDear There females may be treated, according to this technique; the 



^^ ' . diagram illustrates an experiment in which males are treated. 



are in Drosophila at it should be noted that, in this experiment, use is made of two 



present a 



stocks 



such tests possible. The first, and the most widely used, is Muller's 



CI B stock described in detail in his 1928 paper (107), which contains 



a general discussion of the technique of measuring mutation rate. 



number of 'dominant marking genes {.Cy and A, which may be any one 

 of a number of different dominants). 



which make 



