25-3 



THE YEAST CELL 



important fact confirming the view that No. 6040-43 is not an ille- 

 gitimate hybrid of No. 5735 is the presence of one non-fermenter 

 of sucrose in the tetrad. I have pointed out the problems involved 

 in introducing the sucrose gene into the stock and its appearance 

 in this tetrad is proof of its truly hybrid origin. Whatever the cause 

 for the high frequency of dominants (or synthesizers) it is clear (1) 

 that they could not have been differentiated from other phenotypically 

 similar cultures except on the basis of their origin in the same as- 

 cus and therefore are only detectable by tetrad analysis and are un- 

 detectable by any other means, and (2) that misdivisions are not 



WHITE pn A AD IN PY TH 



WHITE PNiad.IN PY a th 



PINK PN ADyin py th 



PINK pn ad In py i TH 



WHITE PN AD IN PY TH 

 PINK pn ad In py^TH 

 PINK pnA(ad)Aln py 



1^1 



PINK PN^ ad ♦IN PY ^th 



th 



WHITE PN AD IN PY TH 



WHITE PN AD IN PY TH 



PINK pn ad In py th 



PINK pp ad In py th 



WHITE PN AD IN PY TH 

 PINK pn ad In py th 



PINK pnA(ad; IN PY TH 

 PINK PN '*' ad In py th 



Fig. 25-2 Four-strand Crossing Over Analysis of Four Sample 

 Asci. Since the Mating was PN AD IN PY TH the first ascus is a 



pn ad in py th 

 non-crossover ascus in which no detected exchanges occurred. In 

 the second ascus, three exchanges occurred at the indicated places. 

 The first pair constitute a 3-strand exchange and the second pair 

 also constitute a 3-strand exchange. The details of analysis of 

 this type have been fully described by Lindegren and Lindegren 

 (1942). The third ascus is one of those containing 3 pink cultures. 

 The circled ad attained full growth in 5 days and is apparently the 

 degraded AD. Therefore, a 2-strand exchange occurred followed by a 

 3-strand exchange at the points indicated. The fourth ascus is the 

 other with 3 pink cultures and a simple exchange occurred in this 

 ascus in the PN-AD region. 



likely to be involved since they would produce tetrads in which some 

 members would be non-viable and in these tetrads every member 

 was viable. Since it is clear that the ascus is truly hybrid one can 

 only conclude that a transfer of chromosomal material has occurred 

 changing the weakly synthetic (recessive) alleles into dominants. 

 The fact that several alleles were affected in this ascus suggests 



