IRWIN I. OSTER 



y sc^' In49 sc^ which had been y sc^' In49 sc^ 



p 1 !! fertiHzed by her ^ ,^ 



**:!«**!(:** + :}:***** ^ brothcTS of the *************** 



y 



B 



composition: * ^^ • * ^^ ^ 



(yellow, Bar eyes) (non-yellow) 



Fg absence of non-yellow, Bar eyed males = sex-linked recessive lethal 

 mutation of paternal origin. 



The use of Bar eyes (B) to mark the irradiated sex-chromosomes makes 

 the detection of sex-linked recessive lethals fairly easy. The scoring of 

 translocations is simplified by using the easily discernible markers, brown 

 (bw) located on chromosome II, scarlet (st), and the peach allelomorph of 

 pink (pP) which are both located on chromosome III. The eyes of flies 

 heterozygous for these markers appear brick-red in colour while the eyes of 

 those homozygous for brown and scarlet are white. These two classes present 

 a strong contrast with either of the recombinants, namely, brown, or scarlet 

 with the peach allelomorph of pink, which look brown or orange, respectively. 

 In order to facilitate the work further, the F^ males and females can be 

 allowed to mate with each other for at least two days before being separated ; 

 this permits one to carry out the tests for lethals by placing one such in- 

 seminated female in a vial rather than having to collect new males with 

 which to mate them as is usually done. When the F^ flies are separated they 

 should be counted to deteimine the frequency of any exceptional males 

 which will appear yellow in contrast to the normal class of males expected. 

 The latter have non-yellow bodies since they carry the normal dominant 

 allelomorph (y+) of yellow in the sc^ insertion of their Y chromosome. The 

 yellow males represent cases of loss of the whole or part of the X or Y chromo- 

 some. The translocation tests can be carried out by mating one Fj^ non-yellow 

 male with one virgin female similar to his mother per vial. As these stocks 

 had been also chosen for their high fertility the amount of sterility resulting 

 from these time-saving procedures is practically negligible {i.e. less than 5 per 

 cent). The F^ fertilized females are tested for lethals {i.e. a mutation affecting 

 some vital process (es) resulting in the death of the individual prior to 

 maturity) by examining the Fg offspring for the absence of Bar (B) males 

 which would indicate that a lethal has been induced in the paternal X 

 chromosome; the F^ males are tested for translocations {i.e. a chromosomal 

 disarrangement involving the breakage of two non-homologous chromosomes 

 and their subsequent reunion in a new order) between autosomes II and 

 III by being backcrossed to y sc^' In49 sc^; bw; st pP virgin females and 

 examining the F2 offspring for the absence of independent assortment 

 between brown (bw) on the one hand and the scarlet (st), peach (pP) 

 combination on the other hand, since the former is not linked with the 

 latter in normal flies. All the Fg cultures which do not yield an adequate 

 number of flies to enable one to determine whether or not a lethal or trans- 

 location is present can be retested by repeating the F^ cross with the Fg 

 generation. 



For the experiments on female germ cells, a modified scheme, based on 



255 



