PROBLEMS OF MEASUREMENT OF MUTATION RATES 51 



not make more than a factor of 2 difference, and doesn't change this 

 picture at all. 



This decline prior to 100 cells, I think, is an artifact. An explanation 

 of the decline is that many of the sectors are noticeably compound — 

 just the phenomenon you referred to. Dr. Lederberg. They obviously 

 have the same origin, but they consist of several lines very close to- 

 gether. Wherever possible, those lines were measured separately, but 

 if they had been summed, they would have brought the number of 

 very wide sectors, which are the ones in here (very early), to be 

 higher, and might have brought them up to 4 per cent. 



This may not be the complete explanation, but the compound 

 sectors are actually more frequent than the simple ones. 



Dobzhansky: What proportion of the albino shells show these lines, 

 anyway? 



Atwood: About 60 per cent of them. The distribution of lines is such 

 that the genetic basis is most likely heterogeneous; the rates are so 

 different among individuals. 



Glass: This phenomenon reminds me very strongly of the sectorials 

 analyzed by Brink in maize (3) . 



Atwood: Yes, that's right. I merely put this forth to show that at 

 least here is an example where some sort of genetic change — and I say 

 "genetic" only in the sense that the cells inherit pigment forming 

 ability from their progenitors — has a rather restricted time of occur- 

 rence. Perhaps it will be possible to learn the genetic basis through 

 breeding experiments. 



Neel: Can Dr. Demerec clarify for us how comparable this is to 

 the plant situation? 



Demerec: It seems to me that the kind of behavior described by 

 Atwood is similar to that found in plants, particularly corn, where the 

 growth patterns of leaf tissue and seed pericarp resemble the growth 

 pattern of the mussel shell. Many years ago, I did a considerable 

 amount of work on just this type of genetic change, studying unstable 

 genes in the larkspur. Delphinium ajacis, and in Drosophila virilis (9). 

 In Delphinium two flower-color genes were studied in detail, namely, 

 rose and lavender. Both mutate to the dominant allele which is purple, 

 and the mutant cells show up as purple sectors on a rose or lavender 

 background. Since the color is limited to one layer of epidermal cells 

 in the sepals and petals, and the cells are large enough to be seen 

 under a low-power microscope, it was possible to determine the size 

 of the purple sectors in terms of the number of cells they contained. 

 The purple sectors on the sepals were counted and classified into 



