19-9 THE YEAST CELL 



Cultures 2111, 2148, and 2153 produced the recorded turbidity 

 in the pantothenate -free medium after 48 hours and were discarded. 

 They would doubtless have grown more, this being indicated by the 

 + sign after the turbidity reading. Each ascus produced two cul- 

 tures with a turbidity reading of less than 8 and two with more than 

 160 four days after inoculation. The genes controlling mating type, 

 galactose fermentation, and maltose fermentation also segregated 

 regularly in each of the eight asci. 



Table 19-4 is a pedigree describing the cultures grown from 

 the ascospores dissected from 24 asci. These asci are derived 

 from a hybrid heterozygous for mating type, galactose fermenta- 

 tion, maltose fermentation, and genes affecting the synthesis of 

 paraminobenzoic acid, thiamin, and pyridoxine. The + and - signs 

 indicate whether or not the cultures ferment galactose or maltose, 

 and whether they grow in Burkholder's vitamin-free medium. The 

 readings on the paraminobenzoic- and thiamin-free media were 

 made after 48 hours, while those in the pyridoxine -free media were 

 made after four days. Two of the cultures from each ascus pro- 

 duced practically no growth at the time diagnosis was made. The 

 readings were all recorded numerically just as were the panto- 

 thenate readings shown in Table 19-3, but for the purposes of clari- 

 ty were converted into + and - signs in the table. 



In these selected pedigrees, regular segregation is the rule. 

 In every ascus except Nos. 5 and 22, all characters segregated in 

 a 2:2 ratio. This is adequate proof that the characters are under 

 genie control. In asci 5 and 22, only galactose was affected; the 

 regular segregation of the other characters proves that these asci 

 were truly hybrid. The significance of the irregular segregations 

 will be discussed in detail in Chapters 25 and 26. 



