24-2 



THE YEAST CELL 



types I and II. These are the 4 tetrads in the upper left hand cor- 

 ner of the table. Combinations of second-division segregations of 

 both A/a and B/b produce tjrpes 1:11:111 in the ratio 1:1:2. These 

 are the 16 tetrads in the lower right hand corner of the table. Com- 

 binations of first- and second-division segregations produce only 

 type in tetrads. These are shown in the two remaining blocks - the 

 uppei; right hand and the lower left hand. 



Since we know that second-division segregation represented as 

 a percentage is equal to twice the distance of a locus from the cen- 

 tromere, it is possible to calculate the expected ratios of I:II:in 

 for various loci at different distances from their centromeres. 



Table 24-1 



Checkerboard showing the 36 different types of asci ex- 

 pected from the random segregation of 2 different pairs 

 of alleles. The six types of asci for the A/a alleles 

 are arranged in the first row; the six types for the B/b 

 alleles are in the first column. These arrangements are 

 derived from the types of segregation found in tubular 

 asci like Neurospora (fig, 23-1) when the genes are seg- 

 regated at random because they are farther than 33 units 

 from the centromere. The resulting arrangements -due to 

 the combinat ion of these types is shown in the squares. 

 Since arrangements are not detectable in a spherical as- 

 cus, the type (l, II, or III) is indicated also. There- 

 fore, if both genes are far from the centromere, the cal- 

 culated ratio of I: II: III - 6:6-24, 



