240 The Nature of Gene Mutations 



differs from the others by a different structural component of the 

 gene molecule and that the different forms of the same allele 

 merely vary slightly within the same component. 



That some genes mutate relatively frequently and are called 

 "unstable" should not be interpreted to mean that frequent mu- 

 tability is a hit-or-miss proposition and that genes may sud- 

 denly indulge in a wild orgy of mutation at any time. Such is 

 far from the truth. We have pointed out that the "alpha," 

 "beta," and "gamma" forms mutate at certain stages of the life 

 cycle only. A great regularity in mutation rate is observed at 

 certain stages when the rose-a and lavender-a genes are studied, 

 and the reddish-a gene in Drosophila virilis is found to mutate 

 only at meiosis in heterozygotes. Such regular behavior indi- 

 cates that mutation in mutable genes is a well-ordered process. 

 Mutable genes appear to become unstable only when a certain 

 stage in the development of an organism is attained, and the 

 precise stage appears to vary with different mutable genes. 



Different Mutation Rates in Different Species 



The rate of gene mutation in one species is not necessarily the 

 same as in another. Baur has indicated that it is about 10 per 

 cent in the snapdragon but that most of these mutations produce 

 only very small changes. In Drosophila, Schultz found that the 

 mutation rate of sex-linked lethals was 0.18 per cent, whereas 

 Spencer found the rate of visible mutations to be about 0.005 

 per cent. In maize cultures, a large number of mutant genes 

 has been discovered. The rate varies considerably for individual 

 genes, and some mutations have turned up again and again. In 

 the evening primrose, on the other hand, although the investiga- 

 tions have been extensive, only a few gene mutations have ap- 

 peared and no gene mutation has appeared twice except the juni- 

 folia mutant which arose once in Cobb and Bartlett's cultures 

 of Oenothera pratincola and once in Shull's strains of Oe. La- 

 marckiana. 



Mutation and Environment 



The mutation rate of different genes may be greatly affected 

 by environmental conditions. Muller, for example, found fewer 

 lethal mutations at low temperatures than at high, and the 



