20-4 THE YEAST CELL 



was relatively irregular. The important point is that illegitimate 

 diploidization of a fermenter produced only nonfermenter progeny. 

 By contrast haploid culture 23 produced illegitimate diploids which 

 threw only fermenter progeny (one with irregular cell shape L(?) 

 and one of these in turn (haploid culture 30) produced illegitimate 

 diploids from which twenty haplophase fermenter progeny were re- 

 covered. 



Haplophase culture 23 from which only galactose fermenters 

 were recovered when illegitimate diploids were produced was back- 

 crossed to haplophase 1 which was a nonfermenter. Three asci 

 were dissected, eleven fermenter haplophases were recovered, and 

 one ascospore failed to germinate. Haplophase cultures derived 

 from ascus (57-60) were plated out and individual colonies tested 

 for their ability to ferment; 57 and 58 threw only fermenter des- 

 cendants; 59 threw only nonfermenters, and 60 threw both fer- 

 menters and nonfermenters. 



It is clear from these data that the control of galactose fer- 

 mentation in S. cerevisiae which segregates in a regular Mendel- 

 ian manner when hybridized with the nonfermenter allele originat- 

 ing from S. microellipsoideus behaves in a very irregular manner 

 when hybridized with the nonfermenter allele originating from S. 

 bayanus . 



NON-MENDELIAN INHERITANCE OF ABILITY TO 

 FERMENT MELIBIOSE 



S. cerevisiae is incapable of fermenting melibiose, and its hap- 

 loid segregants fail to ferment this sugar even after continued 

 growth in broth containing melibiose. S. carlsbergensis is capa- 

 ble of fermenting melibiose by an adaptive enzyme (fig. 20-2), as 

 are all its haploid segregants. This is the principal character up- 

 on which S. cerevisiae and S. carlsbergensis are differentiated. 

 Subsequent genetic analysis established the fact that the haploid 

 cultures of S. carlsbergensis carried only a single gene control- 

 ling the fermentation of melibiose. Hybrids between these two 

 species would therefore be expected to produce two fermenter and 

 2 nonfermenter spores per ascus. None of the first seven asci dis- 

 sected from the hybrid, S. carlsbergensis haplophase (No. 7) by S. 

 cerevisiae haplophase (No. 4) (Table 20-2), were of the expected 

 type. Three were Me Me Me Me, two were Me Me Me me. A back- 

 cross of one of these to the nonfermenter haploid No. 4 produced 

 regularly segregating asci and the back-cross 5 x 23 did the same. 

 However, when 20 and 23 were mated (Me/Me) several nonfermen- 

 ter progeny were obtained. No fermenters were obtained, however, 

 from the me/me crosses. The unconventional behavior of the mem- 

 bers of this pedigree was first interpreted as the result of multi- 

 ple genes controlling melibiose fermentation and is considered by 



