268 CHROMOSOMES IN HEREDITY: MECHANICAL 



D., 1936 d, on Chorthippus, cf. Ch. XII). Such association probably 

 prevents crossing-over, and merely reduces the total chiasma- 

 frequency of the hybrid. 



The later behaviour which characterises a dyscentric hybrid 

 appears only when crossing-over has taken place between the 

 dislocated {i.e., relatively inverted) segments. This behaviour 

 depends on the number of crossings-over and their relationships 

 within the dislocated segments and proodmal to them, i.e., between 

 them and their centromeres (Richardson, 1936, D., 1936 d). 



The result of crossing-over is to produce chiasmata which may be 

 seen to be inverted, either directly (in Chorthippus and Pceonia) or 

 by the inequality of the corresponding arms (as in Tulipa). Where 

 two chiasmata are formed in the inversion they are no doubt liable 

 to move apart, owing to greater repulsion within the loop than 

 outside it. 



At anaphase the behaviour depends on the fact that crossing-over 

 between two dislocated chromatids gives one dicentric chromatid 

 (with two centromeres) or loop chromatid, and one acentric chroma- 

 tid (with no centromere). The dicentric chromatid makes a bridge 

 at the first, and the loop chromatid at the second division ; the 

 acentric chromatid makes a passive fragment. 



The five kinds of separation found at first anaphase in dyscentric 

 hybrids without duplication within one chromosome are as follows 

 (Fig. 91) :— 



1. Normal separation, possibly delayed by exceptional tension : 

 with reciprocal chiasmata within the inversion, whatever chiasmata 

 are proximal to the inversion. 



2. A chromatid bridge and a fragment : with a single chiasma in 

 the inversion, or with two disparate chiasmata in the inversion, or 

 with a chiasma proximal to the inversion, which is comparate 

 (reciprocal or complementary) with regard to chiasmata in the 

 inversion. 



3. Two chromatid bridges and two fragments : with two comple- 

 mentary chiasmata in the inversion. 



4. A loop chromatid, two normal chromatids and a fragment : with 

 a chiasma proximal to the inversion which is disparate with respect 

 to a chiasma, or chiasmata of Type 2, in the inversion. 



