15-6 THE YEAST CELL 



Reaume. Similar white segregants arising in the pink pedigree 

 were subjected to genetical analysis. 



The hybrids of one of the exceptional adenine -deficient methio- 

 nine sufficient (ad MT) white cultures (obtained among the 46 asci 

 from the first hybrid in Table 15-1 by standard pinks, ad(P)MT, 

 produced 21 asci containing 4 pink cultures each. Eight asci from 

 a hybrid of the exceptional white (ad MT) by a standard adenine - 

 dependent, methionine-dependent white produced two pink and two 

 white cultures per ascus; the white cultures were methionine-de- 

 pendent. Six asci of an exceptional white (ad MT) by an adenine - 

 independent, methionine-dependent white produced the progeny that 

 would be expected if the exceptional white were a normal pink. This 

 analysis indicates that the inability of these exceptional adenine - 

 dependent, methionine -independent cultures to produce the pink pig- 

 ment was due to a loss which is restored following hybridization. 



The hypothesis which I have invoked to explain the occurrence 

 of the "mutation" from pink to white and its restoration on out- 

 crossing depends upon the general theory of heredity presented in 

 chapter 28. This theory proposes that the gene produces its effect 

 by initiating the synthesis of a gene -product which passes into the 

 cytoplasm; maintenance of a specific character depends upon a 

 specific gene -product-to-gene equilibrium. Pink color depends 

 upon the presence of the two genes ad and MT and gene -product x. 

 The ad gene indicates adenine -dependence and, therefore, the in- 

 ability of the organism to complete the synthesis of adenine, re- 

 sulting presumably in the accumulation of a precursor of adenine 

 in the cell. In the presence of methionine the precursor unites with 

 the gene-product (x) of sonie non-allelic gene (see fig. 28-1) pro- 

 ducing the pink color; this drastically diminishes the concentration 

 of gene -product x. The metabolism of the cell, which is now white 

 because gene -product x is no longer present in the required amount, 

 becomes stabilized at this new low gene -product-to-gene equilibrium 

 (for gene x) and the clone continues to multiply at the new low level. 

 "Mutation" of pink to white is frequent for continuous production 

 of the adenine precursor frequently diminishes gene -product x and 

 results in "running out" of the pink character. U the normal gene- 

 product-to-gene equilibrium (for gene x) is present in the stock to 

 which the outcross is made, the pink color is automatically restored. 



