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AN INTRODUCTION TO MODERN GENETICS 



of the first terminal chiasmata without the hypothesis of breakage 

 having to be invoked. 



Crossing-over by chiasma breakage is, therefore, a conceivable 

 mechanism, but one whose occurrence has not been demonstrated and 

 is for the above reasons not likely to be general. In his most recent 

 paper Sax^ seems to have himself abandoned the theory. 



6. Compound Crossing-Over and Chromatid Interference 



A bivalent very often contains more than one chiasma, and there are 

 several ways in which the successive exchanges of partner may be 



Fig. 63. Compound Crossing-Over and Sequences of Chiasmata. — The 



diagram shows, on the left, the four possible sequences of two chiasmata in a 

 bivalent consisting of four chromatids. R reciprocal, C complementary, D1 and 

 D2 the two types of disparate. At the right are given the corresponding chromatids 

 after crossing over; the non-cross-over chromatids are labelled N, the single 

 cross-overs S and the doubles D. 



related to one another. The sequences of chiasmata are said to give rise 

 to compound crossing-over; the term double cross-over is primarily 

 derived from breeding, and implies that two cross-overs have occurred 

 in the same sequence of genes, and we shall see that only some of the 

 chromatids resulting from compound cross-overs are actually double 

 cross-overs in this sense. 



Compound crossing-over can be observed cytologically as sequences 



1 Sax 1936. 



