Linkage and Crossing Over Between Genes 



125 



follow in Figure 9-9 the genetic conse- 

 quences of a single crossing over between 

 the loci under study. Since only one of 

 the seven pairs of chromosomes present is 

 being traced, the others were omitted from 

 the figure. As shown, a single crossing over 

 in the four-strand stage produces two cross- 

 over and two noncrossover meiotic prod- 

 ucts. On the other hand, a crossing over 

 in the two-strand stage (in the topmost nu- 

 cleus) would produce only crossover meio- 

 tic products. 



When numerous asci of a particular di- 

 hybrid for linked genes were tested, 90% 

 had all eight spores noncrossovers for the 

 two loci; in the remaining 10%, exactly 

 four of their eight ascospores were cross- 

 overs. In other words, never were all eight 

 spores from a single sac crossovers. This 

 fact demonstrates conclusively that crossing 

 over occurs only in the four-strand stage, 

 as depicted in Figure 9-9 and Figure 9-10. 



It has already been implied that chiasma 

 formation is a normal part of meiosis (p. 

 16). The chiasma prevents the premature 

 separation of dyads by holding them to- 

 gether as a tetrad until anaphase I. (Usu- 

 ally at least one, and as many as six chi- 

 asmata occur per tetrad.) Therefore, the 

 crossing over that subsequently leads to the 

 useful chiasma is also a normal part of mei- 

 osis. 



Since chiasmata are found at numerous 

 positions along a chromosome, it seems rea- 

 sonable to suggest that the greater the dis- 

 tance between two loci, the greater will be 

 the chance for a crossing over to occur be- 

 tween them, and the greater will be the 

 frequency of crossovers for them. Con- 

 versely, the closer two loci are, the smaller 

 will be the chance that crossing over oc- 

 curs between them, and the smaller will be 

 the frequency of crossovers for them. Ac- 

 cording to this view, the frequency of cross- 

 overs can be used as an indication of the 



relative distances between loci. (The re- 

 sults presented in Figure 9-5 should now 

 have additional meaning for us.) 



In the particular Neurospora test men- 

 tioned, no crossing over occurred in 90% 

 of spore sacs in the genetically marked re- 

 gion (a-b). These sacs produced 90% of 

 the total number of spores and carried only 

 parental, noncrossover genotypes. From 

 the 10% of spore sacs which did undergo 



TETRAD 



FIRST 



MEIOTIC 



DIVISION 



8 Spores 



8 Spores 



figure 9-10. Arrangement of spores in the 

 Neurospora ascus when segregation occurs at 

 the first meiotic division {left) and at the 

 second meiotic division {right), as determined 

 by the absence and presence, respectively, of a 

 chiasma between the segregating genes and the 

 centromere. 



