208 Miscellaneous Linkage Topics 



of partial chiasmatypy more nearly represents the correct situ- 

 ation. 



The older, or classical theory, assumes that crossing over does 

 not produce chiasmata, but that it is caused by chiasmata. Ac- 

 cording to this theory, when the four twisted chromatids open 

 out at diplotene they are still unbroken, and they open out in 

 such a manner that sister and nonsister threads are together 



Classical Partial Chiasmatype 



Theory Theory 



One 

 chiasma: 



Three 

 chiasma 



"^ ^^^^^^ ^^^^^ 



a 



Fig. 64. The interpretation of chiasmata according to the classical 

 theory (a) and the theory of partial chiasmatypy (h). 



alternately in successive loops. Beginning at the centromere, 

 sister threads are together; if there is a chiasma on each side of 

 the centromere, nonsister threads would be together on the oppo- 

 site sides of each chiasma (Fig. 64a). These chiasmata produce 

 strains as the paired threads repel one another, and as a result 

 of the strains, some or all of the chiasmata break, producing 

 crossovers at the breaks. As the theory of partial chiasmatypy 

 is more generally accepted today, a detailed discussion of the 

 merits of the two theories and of the various arguments in sup- 

 port of each is unnecessary. 



Exactness of Crossing Over 



One of the most significant features of crossing over is the 

 exactness with which it occurs in the two chromatids which 

 cross over. This feature of crossing over cannot be emphasized 

 too strongly because it is this fact that makes genetic crossing 

 over so regular and permits the construction of linkage maps. 



