Mutation Frequency 



259 



occurred early in the development of the organ, the resulting 

 spot is large; but if it occurred late, the spot is small. Three 

 plants having this frequently mutating gene were studied. Four 

 cuttings were made and rooted for each plant, and one cutting 

 from each plant was allowed to develop at 25° C, one at 30° C, 

 one at 35° C, and one at 40° C. Counts were made of the num- 

 ber of spots on each plant, and the mutation rate was then 

 calculated per unit of stem length, per cell and per unit of time. 



TABLE 7 



Calculated Number of Mutations per Centimeter of a Portulaca 

 Stem 2 mm Thick, and per Cell at Four Different Temperatures 



(Rearranged from Faberge and Beale in the Journal of Genetics.) 



25° C 



per cm 



per cell 



30° C 



per cm 



per cell 



35° C 



per cm 



per cell 



40° C 



per cm 



per cell 



Plant 1 

 Plant 2 

 Plant 3 



2.89 

 2.92 

 2.75 



1.70 

 2.42 

 3.45 



2.05 

 2.39 

 2.22 



1.98 

 2.14 

 2.59 



1.07 

 1.09 

 0.86 



1.25 

 1.28 

 0.90 



0.54 

 0.60 



0.96 

 0.71 



not scored 



The results obtained per unit of length and per cell are listed 

 in Table 7. For this gene, increase in temperature results in a 

 decrease in the number of mutations per unit of length, per cell 

 and per unit of time. 



Another instance in which an unstable gene mutates less fre- 

 quently at a higher temperature than at a lower has been re- 

 ported by Rhoades. In the last chapter we mentioned that 

 Rhoades had discovered a gene, Dt, in maize that causes the 

 normally stable gene, a, to become unstable and to mutate fre- 

 quently to the dominant allele. A, thus producing colored spots 

 in an otherwise colorless aleurone and dominant stripes in the 

 pericarp and in the stems and leaves. When such a Dt plants 

 were raised at 15.5° C, an average of 41.8 mutations was ob- 

 served per seed; but when other members of the same strain 

 were raised at 27° C, the average was only 9.3. 



