350 PERMANENT HYBRIDS 



Gairdner and D., 193 1), in Campanula persicifolia. In polyploid 

 Oenothera there are therefore multiple chiasmata, as in Tradescantia, 

 Rosa, and Primula (Catcheside, 1933). 



The question now arises, if chiasmata are formed, and crossing- 

 over therefore occurs between each pair of segments : where are 

 the differences situated which constitute the complexes and between 

 which no crossing-over, and therefore no pairing, occurs ? Clearly 

 they must lie in the neighbourhood of the centromere, for all associa- 

 tion must take place distally to them if they are not first to arrest 

 terminaHsation and, secondly, to cross-over to opposite chromo- 

 somes, in a word there must be proximal differential segments 



(D., 1931). 



It has now been found cytologically that whenever a pair of 

 chromosomes suffers two interchanges these never coincide with 

 one another or with the centromere (Ch. V). The first result of 

 this property is that in a ring of four chromosomes there will 

 necessarily be an interstitial segment lying between the point of 

 interchange and the centromere in each chromosome. In such a 

 ring crossing-over can occur between the interstitial segments 

 (Fig. 107, Sutton, 1936). But when it occurs half the chromatids 

 produced, with either of the possible segregations of the four 

 chromatids, will be non-disjunctional. There will be what Sansome 

 (1933) describes as " chromatid non-disjunction." This dis- 

 advantage, together with the failure of normal pairing seen near 

 the change of partner at pachytene by McClintock (1933), will 

 reduce the amount of crossing-over, i.e., strengthen the linkage of 

 genes in these interstitial segments with the centromeres which 

 pass to opposite poles in the two pairs of chromosomes. 



When rings larger than four arise from successive interchanges 

 another special segmental property will appear : differential seg- 

 ments in two chromosomes will lie between terminal pairing segments 

 which do not continue their homologies (D., 1931, 1933 ; Sansome, 

 1932). These segments may include the centromeres or not (Fig. 

 106). So long as the type is preserved, crossing-over is suppressed 

 in these segments. Any differences arising in differential segments 

 in opposite sets, whether by gene mutation or by further structural 

 change, will therefore belong to a complex inherited as a unit. 



