Mutation Frequency 257 



cross of Fig. 77. Of the sixteen possibilities from this cross, 

 three die because they are homozygous for Cy, three die because 

 they are SbSb, and one is lethal because it is homozygous for 

 both Cy and Sb. All the S and H types are discarded, and only 

 the viable Cy Sb flies are retained. All have one treated chromo- 

 some from each pair, and the treated chromosomes are the same 

 in all these flies. 



For the final cross in the test, two Cy Sb flies heterozygous 

 for the same two treated chromosomes are mated together. Six- 

 teen theoretical types of offspring are possible but those homo- 

 zygous for Cy, Sb, or both never appear because these genes are 

 homozygous lethals. If no new lethal mutations had been pro- 

 duced in the original treated stock, one-ninth of the viable off- 

 spring would have two treated chromosomes of both pairs and 

 would be phenotypically wild type; two-ninths would have two 

 identical treated second chromosomes but one untreated member 

 of chromosome III, and would be phenotypically + Sb; two- 

 ninths would have a marker chromosome II but two treated 

 third chromosomes and would be Cy + phenotypically; the re- 

 maining four-ninths would have one marker chromosome of each 

 pair and would be Cy Sb. This last type is always present, but 

 some of the other types are sometimes missing and their absence 

 is the key to the test. If a lethal had been induced in chromo- 

 some II, all flies that had two treated members of this pair 

 would fail to survive because they would be homozygous for 

 that lethal. In such a case two-thirds of the surviving offspring 

 of the third cross would be Cy Sb and one-third Cy +. Simi- 

 larly, if a lethal had been induced in chromosome III, two-thirds 

 of the surviving offspring would be Cy Sb and one-third would 

 be H- Sb. If, however, a lethal had been induced in both chro- 

 mosomes, only the Cy Sb offspring of the third cross would be 

 found. By testing a large number of treated chromosomes, an 

 estimate of the minimum number of induced lethals can be 

 obtained. 



Mutation Frequency 



When tests were carried out on the first three chromosomes 

 of a Florida stock of Drosophila melanog aster, it was found that 

 at 25° C, the temperature which is most suitable for raising 

 Drosophila melanogaster. 1.09 per cent of lethals were produced 



