264 CHROMOSOMES IN HEREDITY : MECHANICAL 



heterozygotes (D., 1936 a). It should be noted that where inter- 

 change occurs between " rod-shaped " chromosomes having short 

 arms with no chiasmata in them, a ring ot four can be formed only 

 when there is crossing-over in this interstitial segment (Fig. 89). 



When we come to consider rings of six and larger numbers of 

 chromosomes, a further complication arises. Owing to the negligible 

 chance of two interchanges coinciding there will always be two or 



B B 



Fig. 89. — Diagram showing the genetic relationship to be inferred 

 in the chromatids of the ring of four found in Trimerotropis 

 citrina by Carothers (1931). The arrows indicate the spindle 

 attachments, which are terminal. The pairs in the homologous 

 segments (A and A, C and C) being closest together are lying 

 axially in the spindle. The fact that the B and D segments are 

 lying equatorially proves that there is a chiasma in A and in C. 

 Since B is not homologous with D nor C with A, there must have 

 been crossing-over between the centromeres and the points of 

 interchange, as shown in the diagram, unless chromatids of 

 opposite chromosomes are associated at the centromere. This 

 is associated with the formation of a chiasma in each case. 



Note. — This ring, unlike the normal type, will have the 

 second division reductional and the two chromosomes dividing 

 at random will give 50 per cent, of non-disjunctional progeny. 



more chromosomes in such rings with interstitial segments which 

 lie between end segments neither of which is homologous with the 

 end segments of the homologous interstitial segment. Such seg- 

 ments are differential segments (D., 1936 a), and if they cross over 

 they produce (secondarily) a reverse segmental interchange which 

 will reduce the size of the ring in the progeny (Sansome, 1933, 

 Brink and Cooper, 1935). In Pisum such crossing-over is frequent, 

 in Campanula on the other hand, it is known only by its genetical 



