THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES IO9 



The nature of these chromosomes can be recognized by their effect 

 on the balance of the organism (p. 172), but even more definitely by 

 their association in meiosis.^ In secondaries associations in rings of 

 three may be found, and, even more strikingly, the two abed, abed 

 chromosomes may associate, while the abba chromosome associates 

 with itself to form a ring. This association must be due to the forma- 

 tion of chiasmata, which are subsequently completely terminahzed, 

 and the fact that here the two ends of a single chromosome pair with 

 one another is a remarkable demonstration that zygotene pairing is a 

 relation between individual loci and not between two chromosomes as 

 wholes (p. 99). In tertiary trisomies associations of five chromosomes 

 are possible and are sometimes found. 



2. Segmental Interehange 



The name segmental interchange has been given to the phenomenon 

 of mutual translocation by which chromosomes abed, efgh become ebed, 

 afgh. The formation of such mutual interchanges may be supposed to 

 be by crossing-over of non-homologous chromosomes or by breakage 

 of two chromosomes which happen to be lying against each other in a 

 mitosis. The importance of the phenomenon arises from the peculiar 

 results to be expected m the hybrid between races with the original and 

 with the interchanged segments. Such a hybrid will contain chromo- 

 somes abed, efgh, ebed, afgh, which can pair and be associated in a ring 

 of four, abed, dcbe, efgh, hgfa. Examples of this have been found in 

 several plants (Zea, Datura, Pisum) and in Drosophila and probably 

 Orthoptera.2 Such hybrids are spoken of as interchange hetero- 

 zygotes. 



The segregation from such rings of four or more chromosomes is in 

 most organisms fairly irregular, since the rings are usually orientated at 

 random on the metaphase plate. The irregular gametes with redupU- 

 cated or deficient segments often fail to function. Regular arrangement 

 is found in Oenothera where the chiasmata are completely terminahzed 

 and the centromeres median. 



Chromosome complements of this type have been developed in 

 nature as a method of evolution of true-breeding organisms, which are 

 nevertheless fundamentally hybrid. The most remarkable example of 

 this rather exceptional method of evolutionary divergence is in the 

 genus Oenothera. Here we find, not only separate races which give 

 interchange heterozygotes when crossed (as in Pisum and Datura) but 



^ Cf. Belling 1927. - Rev. Darlington 1937. 



