258 The Induction of Gene Mutations 



in chromosome I, 1.75 per cent in chromosome II, and 2.56 per 

 cent in chromosome III. These figures show that the percentage 

 of lethals is not the same in all chromosomes, but it should hardly 

 be expected since some chromosomes are larger than others. It 

 is very interesting to note that the frequency of mutations in 

 chromosomes I and II stands in about the same ratio to one 

 another as their respective number of genes. This relationship 

 does not hold for chromosome III, however, as almost 50 per 

 cent more lethals are found in III than in II, whereas chromo- 

 some III has only about 10 per cent more genes. These per- 

 centages show that when all conditions of the environment are 

 the same, the frequency of mutations is not always propor- 

 tional to the number of genes in a chromosome, indicating, 

 probably, that some genes tend to mutate more frequently than 

 others. 



Studies were made on the number of lethal mutations in chro- 

 mosomes I and II for different constant temperatures from 8° to 

 31° C. In both chromosomes the percentage of lethals increases 

 with the temperature. At the optimum temperature (23° to 

 25° C), the percentage of mutations in chromosome II was 0.82, 

 at 28° it was 0.95, and at 31° it was 2.77. Below 23°, no muta- 

 tions were observed. These results on constantly applied tem- 

 peratures are consistent with many other similar studies but 

 are very different from the results produced by temperature 

 shocks, that is, by sudden short exposures to higher or lower 

 temperatures. With temperature shocks, the number of muta- 

 tions is greatly increased by sudden exposure to either high or 

 low temperatures. The data for constant temperatures indicate 

 a Qio of about 5, or for every 10° rise in temperature the num- 

 ber of mutations is multiplied fivefold. A Qio (temperature 

 coefficient) of this value tends to indicate that these mutations 

 are probably the result of biochemical reactions. 



An increase in temperature does not always produce an in- 

 crease in the rate of spontaneous mutation and, at times, has 

 actually produced a decrease. Faberge and Beale found that 

 the unstable gene for colored spots in Portulaca grandiflora mu- 

 tated less frequently the higher the temperature. This gene 

 produces well-defined spots or streaks of magenta in the epi- 

 dermis of the petals, stems, and leaves. Each colored spot is 

 the result of a single mutation in one cell. If this mutation 



