UNITS OF RECOMBINATION 



interpreted as due to inversions, a view later confirmed by the 

 observed formation of loops in the polytene chromosomes. 



<i<S 66 66 66 



B b 



Mill 



99 99 



A : 



B bjl D 



I I ' 1 1 ;• 1 1 1 1 1 1 



C ^{{ F 



MM n 



H 



H F 



c F 



2 AbB-BdC-CdD-DbA- 

 A 



B ?.? D 



i I I I I I N 1 M 



AbB-BiC-CfE-EfF-FxD-DbA- 



3C 





AS- BC-CE- EG-GH-HF- FD- DA 



Fig. 32. — Pachytene arrangement, showing how rings can be buUt up by simple 

 interchange in various ways, but always giving interstitial segments as well as pairing 

 teniiiiial segments (united by cross lines). The centromeres are shown as circles; the 

 terminal segments are marked by block capital letters: (i) four pairs; (2) ring of four 

 (with two pairs of interstitial segments marked by small letters corresponding to the 

 whole arms from which they are derived), and two pairs; (3) a ring of six, with two 

 pairs of interstitial segments {b and f) and a differential segment (.v) and one pair of 

 chromosomes GH; (4) and (5) two ways of producing a ring of eight by interchange 

 with GH from the ring of six. The two configurations have the same formula for 

 pairing segments, but different arrangements and numbers of interstitial segments 

 (from Darlington, 1936). 



Inversion, indeed structural change in general, thus reduces the 

 number of recombining units and creates, where the homozygotes 

 are viable, a new super-unit — a super-gene giving a 1:2:1 



133 



