4o6 



BREAKDOWN OF GENETIC SYSTEMS 



It is easier, therefore, to suppose that the failure of chiasmata 

 springs from a failure of true pachytene pairing, such as has been 

 determined with ordinary localisation, rather than as a new un- 

 explained abnormality of chiasma-formation. Thus pachytene 

 pairing may begin normally in these plants, but be interrupted as 



Pre-Meiohc 

 Mifbsis: 



Packyte/\e: 



Cf\ias/j\a-' 

 For/r\at/o/\: 



A^a.I: 



Genotypic 

 Abnormal! ty 



NORMAL 



\ 





Vv 



A.ajr(g) © ^'* 



Poif-Mdotic 



\ \ \ 



Mitdiiy. m. 



/ \ J^ \ 



/ / \ J^^ 





/ \ 



oK 



Nor\- RehKJction Deafk Polymifbm Rchucfor^ 



Structural 

 Hybrid/ty 





@ 



\\ 



< 



\i^ 



\ 



\ / 



Deork 



Noa-Rth\jct'ior\ 



Fig. 124. — Diagram to show the differences between abnormahties 

 of meiosis due to genotypic and structural causes (c/. Table 62). 



it is with localisation by division of the chromosomes. It will 

 then continue as torsion pairing in the intercalary unpaired regions 

 but, owing to the chromosomes having already divided, will give no 

 chiasmata in these parts. It will be effectively incomplete. As 

 we shall see later, there is reason to suppose that paired chromo- 



