26-29 THE YEAST CELL 



Table 26-14 (Continued) 



452 ? G MG W 



453 ? gg^ mg W 



454 ? g MG W 



455 ? G MG W 



456 a g mg W 



457 a G MG P 



458 a g^^ MG W 



459 a G mg P 



The appearance of gg cultures is interpreted to indicate that the 

 G stock (derived from gg) has the ability to convert g cultures 

 to gg; i.e., increases the rate of mutation of g to G. 



These analyses prove that slow fermenter populations comprise 

 mixtures of original g segregants and G fermenter mutants arising 

 from them. The acquisition of the ability for rapid fermentation by 

 the population is not due to cytoplasmic modifications but to muta- 

 tion and selection of the G mutant from the originally g stock. The 

 fact that the G (original) x G (derived) mating produced no nonfer- 

 menters suggests that these genes are at the same locus and that 

 the mutation was a back mutation of g to G. 



fflGH g TO G MUTATION RATE AMONG THE 

 DESCENDENTS OF gg PARENTS 



The mating of the derived G to a stable g parent cited above 

 indicates that capacity for mutability of g to G is readily transmit- 

 ted through the derived parent. Since the distinction between g and 

 gg phenotypes merely depends upon the chance of mutation occurr- 

 ing in the latter and its failure to occur in the former, an extended 

 analysis was made to determine if the g and gg clones appearing in 

 the same pedigree were distinguishable genotypically. When a mat- 

 ing is made of a gg by a G clone copulations occur between g and G 

 cells, for mutant G cells are only present in the mating tubes with 

 a frequency of 0.5 x 10-'^. The gg phenotype is only apparent when 

 a test of the segregant is made in galactose broth. 



