254 The Induction oj Gene Mutations 



would be reduced. Since more eggs would remain unfertilized 

 than normally, and since they w^ould produce males, the de- 

 crease in females would be accompanied by an increase in the 

 males among the offspring. If zygote lethals were produced, 

 there would be a decrease in the females, but there would be 

 no accompanying increase in the males, and the number of off- 

 spring would be fewer than in a normal progeny. In a series of 

 experiments, females were crossed with X-rayed males, and in 

 many of the experiments only males were produced. As the 

 number of offspring is only about half the normal number, there 

 is good evidence that dominant zygote lethals were induced. In 

 some families, females were produced, but they were much fewer 

 than in a normal population; and again the number of offspring 

 was considerably below the normal numbers. Whether these 

 dominant zygotic lethals are true gene mutations or chromosomal 

 aberrations is not easy to determine, as the effect would be the 

 same in either case. 



Temperature 



Numerous studies have been made on the effect of tempera- 

 ture on the production of mutations in Drosophila inelanog aster. 

 A number of geneticists have worked on this problem, but prob- 

 ably the most significant work has been done by Plough, Child, 

 and Ives, and their students. When they raised larvae at vari- 

 ous temperatures from 4° C to just below the killing temperature 

 they found that a definite increase in the number of spontaneous 

 lethal mutations was produced by temperatures above the nor- 

 mal. In studying lethal mutations at different temperatures, 

 the CIB method previously described can be used for the X chro- 

 mosome, but another and somewhat more laborious method must 

 be used for chromosome II and chromosome III. 



The various studies on lethal mutations have shown that mu- 

 tation frequency at different temperatures depends in part upon 

 the particular stock or strain of the fruit fly, even though all the 

 stocks are phenotypically wild type. For example, Demerec 

 found that between 22° and 25° C, the range of temperature at 

 which flies are normally raised, the mutation rate in chromosome 

 I of a stock from Florida was 1.09 per cent whereas the rate of 

 a strain from Formosa was only 0.39 per cent and of a par- 



