15-2 THE YEAST CELL 



Three asci produced fewer than the expected number of pink 

 segregants. One contained three white and one pink culture; the 

 pink segregant and one white culture were adenine -dependent. Two 

 asci produced 4 white cultures; two were adenine -dependent and two 

 were adenine -independent. These exceptional adenine -dependent 

 white cultures will be discussed in detail below. 



From one ascus more than the expected number of pink cul- 

 tures was obtained (three pink and one white culture) and the three 

 pink segregants were adenine -dependent. Most of the adenine -de- 

 pendent cultures adapt after four days and grow in the adenine-de- 

 ficient medium. The differences in growth in adenine -deficient 

 and adenine -containing medium are generally diagnostic on the 

 second and third days, but growth of the so-called adenine -depen- 

 dent cultures in the adenine -deficient medium becomes fairly dense 

 on the sixth and seventh day. 



The hybrid was also heterozygous for mating type, galactose-, 

 maltose-, and melibiose -fermentation and there were no exceptions 

 of these characters to Mendelian segregation. 



The hybrid from which the 46 asci were analyzed was homo- 

 zygous for genes controlling the synthesis of methionine, so that 

 relatively adequate amounts of methionine were available to all 

 four segregants. Tatum and Reaume also discovered a methionine - 

 dependent mutant produced by mustard gas treatment and this gene 

 was introduced into the stock by a series of matings. A hybrid 

 heterozygous for adenine -dependence and methionine -dependence 

 was produced. Analysis of 74 asci from the reciprocal crosses 

 ad (P) MT x AD mt (pink x white) and ad mt x AD MT (white 

 X white) revealed that the development of pink color required 

 methionine. The adenine -dependent white cultures were also 

 methionine -dependent and all the adenine -dependent pink cultures 

 were methionine -independent proving that methionine was required 

 for the development of the pink color. 



A doubly heterozygous hybrid produces only three kinds of asci 

 (Chapter 24) and the frequency of these three types can be used to 

 detect linkage of either of the genes with each other or with their 

 respective centromeres. The data in Table 15-1 show that the two 

 reciprocal matings, one producing 35 and the other producing 29 

 asci, both follow the same pattern and the three kinds of asci are 

 present with a total frequency of 14:10:51, which is statistically 

 equivalent to a 1:1:4 ratio, proving that the two genes are not 

 linked. Asci containing 2 pink : 2 white; pink : 4 white; 1 pink: 

 3 white, correspond to the same three categories and reveal that 

 pink is produced as a result of a two-factor interaction. 



Six asci from a pink by pink hybrid produced four pink cultures 

 from each ascus. Three asci dissected from a homozygous adenine - 

 dependent, methionine -dependent hybrid produced only white cultures. 



