THE NATURE OF THE GENE 381 



genes, which can mutate hundreds of times in a single individual, while 

 at the other end of the scale there are certainly genes of which no 

 mutation has been noticed in all the millions of Drosophilas which have 

 been examined. 



Spontaneous mutations may occur in somatic tissues as well as in 

 germinal tissues. It can, of course, only be discovered in the former if 

 the mutant gene is dominant or is sex-linked in the heterogametic sex 

 and therefore has no normal allelomorph to mask it. There is consider- 

 able evidence that certain genes mutate at different rates in different 

 tissues. This is certainly so for some of the highly mutable genes 

 studied by Demerec^ in Drosophila virilis. Moreover, Stadler^ has re- 

 corded the mutation rates of some genes affecting the grains in maize. 



Fig. 153. Frequency of Spontaneous Mutation of some Genes in Maize. — The 



genes affect the grains so that very large numbers can be easily tested. 



(Data of Stadler.) 



Gene Gametes Tested 



R 554.786 



/ 265.391 



Pr 657.102 



Su 1,678,736 



y 1,745,280 



Sh 2,469.285 



Wx 1,503,744 



These genes mutate in germinal tissues at rates which would give 

 several mutations in every plant, if they occurred in somatic tissue, and 

 since such somatic patches are not found, one must suppose that the 

 somatic mutation rates are considerably lower than the germinal. 



The spontaneous mutation rate of a given class of genes (e.g. sex- 

 linked lethals) in a given tissue appears to be constant; each gene is just 

 as likely to mutate at one time as at any other, and the number of 

 mutations is simply proportional to the lapse of time.^ The rate is also 

 dependent on temperature, being higher at higher temperature. This 

 increase in rate occurs even though the length of the life cycle is shorter 

 at higher temperatures. The mutation rate, in fact, has a higher tem- 

 petature coefficient than the developmental processes; its value, cor- 

 rected for the shortening of the Hfe-span, is about 5.^ The mutable 

 genes are an exception to this, showing no great mutation rate (in 



1 Demerec 1933, 1935. 2 Quoted Demerec 1933. 



^ For genetic factors which affect mutation-rate, see Demerec 1937. 

 * Muller 1928. 



