S. SPIEGELMAN AND A. M. CAMPBELL 



positives during division in the absence of substrate. These 

 single-cell analyses confirmed the earlier experiments cited, in 

 demonstrating that abruptly after about five generations cells of 

 the negative phenotype are produced with increasing frequency. 

 The data were uniquely amenable to a probability analysis. It 

 was found that a fully induced positive cell contains in the 

 neighborhood of a hundred particulate units and that the 

 minimal number of particles required for a cell to exhibit the 

 positive phenotype on a test plate is in the neighborhood of one. 

 The probability of a given particle passing into the daughter 

 cell was closely approximated by one half. 



It proved further possible to offer evidence for the 

 extrachromosomal nature of the particles in this particular 

 instance by the comparative study of the transmission of the 

 enzyme-forming capacity during reductional segregation (78). 

 The method used was identical to that employed in a previous 

 study involving the inheritance of the ability to ferment meli- 

 biose (86), with a similar result. The effect of prior induction of 

 (Gg^) heterozygotes to galactose on the segregation of the 

 capacity to form the enzyme was studied. Noninduced hetero- 

 zygotes, with few exceptions, yield a 1 : 1 ratio of positive to 

 negative spores on galactose test plates. However, if the 

 heterozygotes are induced prior to segregation, and the latter 

 occurs in the presence of substrate, all four segregant spores 

 exhibit the positive phenotype in over 90 per cent of the asci 

 dissected. Growth in the absence of substrate for seven to 

 twelve divisions of four such positive spore clones, leads to a 

 reversion of two of them to the negative phenotype, thus re- 

 storing the Mendelian ratio of the phenotype. The fact that 

 homozygous recessives carried through the same procedure do 

 not yield positive spores suggests that the presence of both 

 the dominant gene G and the substrate leads to the uniform pro- 

 duction of the extrachromosomal elements which are necessary 

 for enzyme formation. When, in the course of the segregation, 

 elements are incorporated into the cytoplasm of spores carrying 

 the g^ allele, they are converted to the positive phenotype. The 



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