52 MUTATIONS 



categories ranging from 1 to 4,096 purple cells, each category having 

 double the number of cells of the preceding one. It was assumed that, 

 on the average, a mutation responsible for a sector having twice as 

 many cells as another sector had occurred one cell generation earlier. 

 Theoretically, each category represented mutations that had occurred 

 during the same cell generation. One-cell sectors originated through 

 mutations in the last cell division, two-cell sectors originated in the 

 division before the last, and 4,096 sectors in the twelfth division before 

 the last. Mutations occurring in the early stages of the development of 

 a plant showed up as chimeras with a purple sector extending over 

 several flowers. The data revealed that the rose gene mutated at the 

 same rate throughout twelve divisions of the epidermal cells of the 

 sepals, and that the mutation rate was similar in the earlier stages of 

 the ontogeny of the plant. Depending on the strain, the rate was 

 between 0.22 and 8.1 per 10^ cells per one cell division. The lavender 

 gene, on the other hand, had a high rate of mutability early and late 

 in ontogeny, and during the intermediate stages the rate was very low 

 or the gene remained stable. These results make it seem probable that 

 mutations occur during the multiplication of genes, and, since one- 

 celled sectors were present, they support the hypothesis that gene 

 multiplication is accomplished by the formation of a new gene, by the 

 old one, rather than division of the gene into two equal parts. 



Of the three unstable genes studied in Drosophila virilis, reddish- 

 alpha mutates at the reduction division of heterozygous females only ; 

 magenta-alpha and miniature-alpha mutate in the germ cells and in the 

 somatic tissue only ; and miniature-beta does not revert to the wild type 

 at all, but changes with a low frequency to the more mutable gamma or 

 alpha forms. "It appears as if a mutable gene becomes unstable when- 

 ever the cell reaches a certain stage in the development of the organism 

 where, probably, the chemical properties of the cytoplasm are such 

 that they stimulate the reversions in the genes. Different genes, ap- 

 parently, require different conditions to become mutable" (9). 



Auerbach: I was just going to ask how general you think this is, 

 because in McClintock's cases, you may get what you call a change 

 in state, which changes simply the time of origin of the sectors. 



Atwood: I don't know how general this phenomenon is. This is an 

 example of something that happens in a certain time range, rather 

 early. There are other examples of genetic changes that happen over 

 different time ranges. In this instance the cell division rate during the 

 maximum time of origination is higher than at any later time, so if the 

 process turns out to be mutation, then we would say that this mutation 



