Que$tio?is and Problems 269 



mutant genes. Another possibility has also been suggested that 

 other cell constituents may obtain the energy of these activated 

 molecules and that they may therefore undergo chemical 

 changes; these chemical changes in the medium in which the 

 gene is found may then bring about the mutations. This ''in- 

 direct" theory is compatible with the linear relationship between 

 dosage and mutation frequency and also with the observation 

 that mutation is independent of wave length. If we assume that 

 the cellular environment of the genes is different in different 

 strains of the same species, the different mutation frequencies 

 in such strains can be readily explained by the "indirect" hy- 

 pothesis. 



Basing his conclusions on the behavior of his Dt and a genes 

 in maize, Rhoades has suggested the possibility that an altered 

 cellular environment may be responsible for gene mutation. He 

 considers that the best explanation for the fact that the Dt 

 gene causes the a gene to become highly mutable although it 

 is very stable on a dtdt background is that the Dt gene alters 

 the cellular environment of the a gene chemically. He points out 

 that the Dt gene is specific in its nature since it acts on only 

 one gene whereas the action of X-rays, radium, and temperature 

 is general. Also, short-wave radiation and temperature cause 

 numerous chromosome abnormalities whereas the Dt gene does 

 not. However, the possibility still remains that various fac- 

 tors, both internal and external, that increase mutation rate may 

 act by changing the chemical nature of the gene environment. 



QUESTIONS AND PROBLEMS 



1. Would you expect to have much success in improving a stock of 

 plants by the induction of mutations by radiation? Explain. 



2. Assuming that the average yearly temperature of two regions is 

 the same, would you expect to find more mutations in a region of uni- 

 form temperature or in one in which the temperature fluctuates consid- 

 erably with wide extremes? Why? 



3. What are the phenotypes of the following flies: 



f -h fu f + fu f B fu f BB fu f BB fu 

 f + fu + B + -f B + + B fu + BB fu 



