SEGMENTAL STRUCTURE 343 



(14) {AB CD EF GH KL MN OP 



\/ \/ \/ \/ \/ \/ \ 

 BC DE FG HK LM NO PA) 



It is supposed that this arrangement of segments arose as a result 

 of successive interchange in the ancestry of the plant, as had been 

 inferred in trisomies in Datura (BelUng, 1927). Thus an 

 ancestor with simple pairs, such as that given, would by interchange 

 between B and D give new chromosomes BC and DA, which would 

 form a ring of four with the parental type, as follows : 



(4) + 5(2) {AB CD {EF {GH {KL {MN {OP 



\ / \ 

 BC DA) EF) GH) KL) MN) OP) 



This h3^pothesis explains the constancy of the behaviour of the 

 species, both in heredity and in chromosome association, for each of 

 the complexes is made up in a specific and constant arrangement. 

 The dissimilarity of the complexes is correlated with the dissimilarity 

 in the arrangement of the segments in the two kinds of gametes 

 produced by each species. Just as each plant is the result of the 

 fusion of two complexes, so also is it the result of the fusion of 

 gametes differing in respect of the arrangement of segments in such 

 a way as to give the chromosome ring characteristic of the species. 

 It is a structural hybrid. 



The hypothesis also explains their behaviour on hybridisation, 

 for Cleland and Blakeslee (1930) have been able to predict from the 

 configurations seen in crosses of two species with a third what 

 configurations would be found in a cross between the first two. 

 Their predictions were verified. 



Let us consider the way in which interchange will give rise to 

 ring-formation and complex-inheritance in nature. 



Interchange will always lead to the production of interchange 

 heterozygotes in the first generation just as gene mutation leads to 

 the production of mendelian heterozygotes. These wiU segregate 

 homozygous new and old forms in the next generation unless the 

 interchange is associated with the production of a lethal type. 

 Jn the latter case a permanent interchange heterozygote will arise 



