GENE MUTATION 



Figure 74. The Genes? This is an electron 

 micrograph of a portion of the saHvary gland 

 chromosomes of Drosophila, and the leaf-shaped 

 bodies were once thought to be genes. (From 

 Pease and Baker, Science, V. 109. ) 



genes. If the boundaries of the gene are actually indefinite, then the sig- 

 nificance of such estimates is considerably reduced. 



Whatever their nature, then, there is very abundant evidence that the 

 genes do mutate to produce permanently inheritable alleles. And it is 

 equally evident that this is a source of variability which must be studied 

 for its bearing on evolution. A mutation may be defined as a permanent 

 change in a gene. Alleles exist only because the original wild-type gene 

 has mutated at some time. Like the original gene, the mutant is recognized 

 by the character which it causes. 



Mutation in Nature. Critics of genetics long held that the mutants with 

 which geneticists worked could not be significant, because such mutants 

 were not a really natural phenomenon, but a sort of degenerative effect of 

 laboratory environment. This has been abundantly disproved, for many 

 investigators have found in nature many mutants identical with or similar 

 to those dealt with by laboratory zoologists. The original Drosophila mu- 

 tant (white eyes) was, as a matter of record, captured in nature by the 

 entomologist Lutz at about the same time that it arose in the laboratory 

 stocks of T. H. Morgan. Goldschmidt found many mutants in the wild 

 populations of Drosophila near Berlin. Tschetverikoff inbred the offspring 

 of 239 wild Drosophila melanogaster from southern Russia, and 32 reces- 

 sive mutants segregated out. Dubinin collected the same species in several 

 localities in the Caucasian Mountains. On inbreeding, he found that the 

 incidence of lethal genes varied from to 21.4 per cent in different locah- 

 ties, while visible mutations varied from 3.9 to 33.1 per cent. Many of 

 these had minor effects, such as a slight reduction in size of bristles. Baur 

 studied wild snapdragons {Antirrhinum) and found that about 10 per 

 cent of the plants showed at least one mutant. Dobzhansky showed that 

 in wild populations of Drosophila pseudoohscura 75 per cent of the chro- 

 mosomes showed at least one mutant. Dice has found that wild popula- 

 tions of the deer mouse Peromijscus always have many mutants. It may 

 then be regarded as established that mutation is a normal phenomenon 

 in nature. 



207 



