Fig. 23-1 First- and Second-Division Segregation in 

 Neurospora. 



a. When reduction occurs at the first division, 

 the 4 spores at the upper end and the 4 spores at the lower end 

 of the ascus are different. Reversing the orientation of the 

 first spindle would produce the opposite arrangement; namely, 4 

 white at the top and 4 black at the bottom. 



b. This second-division segregation produces an 

 alternate 2 by 2 arrangement of spores in the ascus and revers- 

 ing the orientation of the second division spindles would reverse 

 the arrangement from top to bottom. 



c. This type of second division segregation pro- 

 duces 4 similar spores in the center of the ascus with two of the 

 opposite types at either end. Reversing the second division 

 spindles would exactly reverse the pattern. This diagram shows 

 that 6 kind of asci can be produced in Neurospora, two character- 

 istic of first-division segregation and 4 characteristic of 

 second-division segregation 



_ o— - 



.ii... 



CHOSSOVtK 



Fig. 23-2 Diagram of crossing-over in Neurospore at the 

 four-strand stage showing how a single cross-over between the 

 centromere and the P/p locus results in second-division segrega- 

 tion of P from p, while the absence of crossovers between the 

 centromere and +/- result in first-division segregation of + 

 from -. 



23-2 



