362 PERMANENT HYBRIDS 



giving two new chromosome types YP and QA, will form a chain of 

 five in the interchange-cum-sex-heterozygote, thus : 



YP-PQ-QA-AXB-B ; 



this will yield two types of gametes YP, QA , B and PQ, AXB. The 



Fig. 113. — Above : first metaphase in (^ Humuhis japonicus XY-^Y^ 

 at left. Below : stages in co-orientation of XY^Y^, some 

 showing exceptional interstitial chiasmata. X 5000. 



homozygous sex will be unaltered by the change, still having the 

 composition : 



PQ-QP, AXB-BXA. 



The chain of four in H. lupulus differs from this in their being 

 no " B " chromosome. 



The sex heterozygote is therefore, apart from other differences, an 

 interchange heterozygote, and the mechanism combines the usual 



