22-15 



THE YEAST CELL 



that these two cultures differed from each other by a single allele; 

 the mating was aB by ab, A mating of unadapted 2154 ( — ) by un- 

 adapted 2151 ( — ) gave 4 asci each of which produced only recessives; 

 this is a homozygous ab by ab cross. A mating of 2154 (+, adapted) 

 by its independent sister 2152 (+) produced six asci containing three 

 independents and one ascus containing two independents. The asci 

 are AB Ab aB ab, and AB AB ab ab respectively. The third type Ab 

 Ab aB aB did not appear. A mating of 2152 (+) by the unadapted 

 2154 ( — ) revealed that 2152 contained only a single dominant allele. 

 A mating of 2152 by 2153 revealed that 2152 and 2153 each carried 

 a single dominant gene and that these genes were alleles since no 

 dependent spores were obtained. 



This analysis makes it possible to assign the genotype aB to 

 2154 (+, adapted) and ab to 2154 unadapted; 2153, 2115, 2114 and 

 2152 are Ab, and 2151 and 2113 are also ab. 



Two of the asci obtained from the mating of 2152 by 2154 adapted 

 were subjected to further analysis shown in the following tabulation. 

 Cultures 4175, 4176, 4177, and 4178, from a single ascus, contained 

 three independent and one dependent clone. 4176 (AB) mated to 2151 

 (ab) dependent produced one ascus containing four independent spores, 

 two asci containing three, and one ascus containing two. This is ob- 

 viously a mating of a double mutant independent clone by a double 

 mutant dependent, AB by ab. Therefore, 4175 is the double mutant 

 independent. Mating of the two other independent clones, 4175 and 

 4178, by dependents, produced asci indicating that 4175 and 4178 

 each contained a single independent allele. 



