Classical Theory of Chiasmata 



207 



on either side of this region. This statement was based on the 

 assumption that crossing over occurs between only two chroma- 

 tids at any one point and would not be true if all four chroma- 

 tids, or if whole chromosomes, crossed over at one point. 



It has been stated that crossing over occurs between one 

 chromatid of one chromosome and one chromatid of the other 

 chromosome. It might seem possible that the two chromatids 

 from the same chromosome might also cross over. The two 

 chromatids derived from the same chromosome are spoken of 



I 



a 



Fig. 63, Crossing over and the resulting chromosomes according to 

 (a) the theory of total chiasmatypy and (fc>) the theory of partial chias- 

 matypy. 



as sister chromatids whereas two chromatids from different ho- 

 mologues are called nonsister chromatids. The evidence indi- 

 cates that crossing over occurs between nonsister chromatids 

 only and not between sister chromatids. 



Classical Theory of Chiasmata 



When seen under the microscope, all four chromatids of a 

 bivalent are normally indistinguishable. According to the par- 

 tial chiasmatype theory, breaks occur at pachytene between two 

 of the four. These two rejoin in a new arrangement, and chias- 

 mata result which are observable when the threads open out 

 into loops at diplotene. According to this theory, every chiasma 

 represents a place where a break occurred at pachytene. As will 

 be seen from Fig. 646, sister threads are always paired and are 

 always found together at each side of a chiasma. Another and 

 older theory has also been suggested which is a basically dif- 

 ferent interpretation of the same observed phenomena. Neither 

 theory can be proved on the basis of visual observation of nor- 

 mal bivalents alone, but the study of a heteromorphic bivalent 

 and of complex interlocking of bivalents shows that the theory 



