382 AN INTRODUCTION TO MODERN GENETICS 



terms of life cycles) at higher temperatures. The significance of this is 

 discussed later (p. 389). 



2. The Induction of Mutation 



The first successful attempt to induce mutations in controlled 

 experiments was made by Muller in 1927, using X-rays on Drosophila. 

 The effect of such radiations in causing an increased mutation rate has 

 since been amply confirmed on other organisms; Timofeeff-Ressovsky 

 lists forty-three species, ranging from Protozoa to the higher plants 

 and animals, in which mutations have been induced. Among the 

 susceptible species, man (and in fact all species) must presumably be 

 included. The sociological results of the exposure of human gonads to 

 high frequency radiation, which is now a not uncommon occurrence in 

 the course of medical treatment or in industrial processes, has hardly 

 yet been considered seriously enough, except in Germany.^ The majority 

 of mutations induced are likely to be both deleterious and recessive; 

 the latter character will prevent them showing in the immediate off- 

 spring of rayed individuals, and may obscure the responsibility of 

 radiation for their occurrence, but their injurious effects will not be so 

 easily overlooked. (From figures given later in this section it will be 

 seen that the magnitude of the effect is that the mutation rate is about 

 doubled by a dose of 30 r units.) 



The fullest data on induced mutations relate to Drosophila, mainly 

 to D. melanogaster. There are two standard genetic techniques used in 

 this work, the CIB and attached-X methods. In the former, males are 

 rayed and crossed to females containing one X chromosome which 

 carries a cross-over suppressor C, a lethal recessive / and the dominant 

 eye-gene Bar. Her Bar daughters must contain the CIB chromosome, 

 and also a rayed X chromosome from their father. They are crossed to 

 any male; half the F2 sons will die (because of the lethal in CIB) while 

 the other half will show any recessive sex-linked gene which has been 

 produced in the rayed X, or will die if a lethal has been produced. This 

 method thus enables one to detect both recessive sex-linked lethals or 

 visibles ; the detection of the former is particularly easy, since they give 

 F2 families containing no males. The attached X method only reveals 

 sex-linked genes with visible effects. Rayed males are crossed with 

 females in which both X chromosomes are attached to one another, 

 and which therefore give 100 per cent non-disjunction; the sons of the 

 cross have received their single X from their father, and will show any 

 1 Stubbe 1934, Pickham 1936. 



