26-3 THE YEAST CELL 



A mating homozygous for g and me produced no fermenter 

 progeny. This is an example of the general rule that homozygous 

 matings of nonfermenters do not produce fermenters. 



One ascus from the doubly heterozygous hybrids (GME x gme) 

 35 X CIA produced a GME GME GME GME ascus. The ascospores 

 from the exceptional ascus (167, 168, 169, 170) were inbred and 

 mated separately to both the recessive and the dominant stocks. 

 When these data were first presented (Lindegren and Lindegren, 

 1946) backcrosses had only been made to recessive stocks and 

 analysis of the hybrids indicated that each of the fermenter pheno- 

 types in the ascus 167 to 170 behaved with certain conspicuous ex- 

 ceptions as dominants. The exceptions are apparent in a summary 

 of the data, table 26-2. When the backcrosses were made to the 

 fermenter parents only very rare nonfermenter offspring were en- 

 countered. Finally, when the intra-ascus matings were made a 

 considerable number of recessive phenotjrpes was encountered. 

 This latter fact was the clearest evidence that we were dealing 

 with irregular inheritance, involving apparent dominants which could 

 either lose their dominant character or imprint their dominant char- 

 acter on recessive individuals. The predominant ascus type pro- 

 duced by mating the original stocks to recessives are two fermenter 

 and two nonfermenter spores per ascus. Those with four fermenters 

 might be illegitimate offspring, but those in which 3:1 ratios occur- 

 red are difficult to interpret on any conventional basis. The higher 

 frequency with which recessives appear in the backcrosses to the 

 fermenter type are evidence of irregular behavior and this is rein- 

 forced by the frequency of recessives in the intra-ascus matings. 



Illegitimates were obtained from culture 170 and showed an ex- 

 ceptionally high frequency of the recessive phenotypes; in this case, 

 only two spores survived per ascus. Another unusual fact in this 

 pedigree was that 167 also produced illegitimate spores which were 

 not analyzed. Culture 169, at first diagnosed as an a mating type 

 culture, behaved in some matings as an a. In spite of the absence 

 of other markers these data were considered convincing evidence 

 of conversion and this view was fully corroborated by the recent 

 work of Mundkur, 1949, in which the phenomenon of conversion is 

 analyzed, in hybrids heterozygous for other genes than G and ME. 



In the backcross of fermenter cultures 167, 168, 169, and 170 

 to the nonfermenter (Table 26-2), each of the matings yielded ex- 

 ceptions to the expected 2:2 ratio per ascus. For example, the 167 

 (G X g) mating yielded 1 GGGG ascus, the 168 (G x g) mating yield- 

 ed 3 GGGG asci, the 169 (G x g) mating yielded 4 GGGg and 1 GGGG, 

 and the 170 (G x g) mating yielded 1 gggg and 1 Gggg ascus. The G 

 genes in 167, 168, and 169 are capable of transforming g genes into 

 phenotypic dominants, but the G gene in 170 is "weak" and often 

 loses its own capacity to ferment. Many of the asci are of the ex- 



