20-6 



THE YEAST CELL 



Spiegelman (1946) to be due to cytoplasmic transfer, but the ex- 

 planation in chapter 26 may fit the facts more adequately. 



Fig. 20—2 Graph Showing the Adaptive Nature of Melibiose 

 Fermentation by S. Carlsbergensis . Glucose— grown culture placed 

 in a Warburg vessel containing 4 per cent melibiose in M/15 

 KHoPO^. Ceil division does not occur during this period so 

 adaptation occurs in a stationary population. 



IRREGULAR RATIOS IN OTHER ORGANISMS 



Irregular ratios, which are frequently encountered by geneti- 

 cists, are generally assumed to result from (1) multiple factor in- 

 heritance (Nillsson-Ehle), (2) suppressors (Bridges) or (3) modi- 

 fying factors. That these genetical mechanisms can produce ir- 

 regular ratios has been amply demonstrated, but the present analy- 

 sis suggests that irregular ratios may arise from other causes as 

 well. The genetical data on which the moderrt conception of the 

 gene is based are intensively selected data. When genetical ex- 

 periments do not give the expected ratios, the worker explores a 

 number of possibilities known to disturb regular segregation of the 

 character, and may eventually resolve his difficulties by discover- 

 ing that multiple factors, or suppressors, or modifying factors, are 

 involved; in the event that he fails to ao so he often discards the 

 material in search for some that is more easily amenable to Men- 

 delian analysis. 



When the progeny of a mating cannot be classified, one cannot 

 determine what effects are responsible for the irregular ratios 

 unless specific tests are made. It is possible, however, that the 

 tests may not have validity because the act of preparing for them 

 (purifying the stocks) may alter the situation to the extent of in- 



