236 The Nature of Gene Mutations 



In laboratory stocks, natural selection is not an important fac- 

 tor; but there are certain technical problems that present diffi- 

 culties when we try to follow the mutation rate of any specific 

 gene. The evidence from mutations in some organisms shows 

 that perhaps the most frequent mutations are those that produce 

 only slight effects. Such small mutations are extremely difficult 

 to detect in wild populations, and even in controlled cultures 

 they make a complete analysis almost impossible. As genes are 

 ordinarily very stable, a determination of the mutation rate of 

 most genes would begin to be accurate only when very large 

 numbers of individuals were raised and examined. Although 

 populations of necessary size would be possible in Drosophila, 

 they would ordinarily be impractical, and they would be almost 

 out of the question for plants and for most animals. In spite 

 of the difficulties, a surprisingly large amount of data has been 

 obtained both from wild populations of Drosophila in parts of 

 Russia and for lethal mutations in the X chromosome of labora- 

 tory stocks of Drosophila melanog aster. 



The data that have been collected show that most genes are 

 very stable, but that gene mutation cannot be considered to be a 

 rare event. The actual mutation rate of a gene depends (1) on 

 the particular gene in question, (2) on the species, (3) on the 

 environment in which the organism is living, and (4) on the 

 entire genetic constitution of the organism. 



Some genes are exceedingly stable; others mutate frequently. 

 A genetic locus cannot be identified unless at least two alleles 

 at that locus are observed. Since estimates of the number of 

 loci in various organisms indicate that most loci have never 

 been discovered, the inference is that most genes are so stable 

 that they do not mutate at all or that they mutate so infre- 

 quently that the few mutants never chanced to be found. From 

 this extreme case, various mutation rates can be found. Stadler 

 showed that in maize this difference may range from the condi- 

 tion at the waxy locus where no mutations were observed in one 

 and a half million tested gametes to the locus of the gene for 

 colored aleurone and plant, R, which mutated at a rate of 492 

 per million tested gametes. Various intermediates were found. 

 At the other extreme from the very stable genes are a number 

 of genes which mutate so frequently that they are known as 

 mutable genes or unstable genes. 



