THE INTEGRATION OF DIFFERENCES 



The Integration of Differences 



We have seen that the hereditary materials, the chromosome com- 

 plement contained within the nucleus, can undergo changes of three 

 types, in number, in structure, and in a residual class of point 

 mutations. The distinction between the structural, or inter-genic, and 

 the intra-genic changes follows inevitably in theory from the fact 

 that the chromosome is a linear arrangement of dissimilar particles. 

 But it cannot always be made in practice owing to the fact that the 

 two types can be combined in one integrated difference. 



We have seen that the combination of the dissimilar chromosomes 

 and parts of chromosomes in a working set constitutes balance. The 

 primary changes that occur in their numbers and in the arrangement 

 of their parts do not upset this balance in vegetative life. But when 

 meiosis takes place these primary changes are broken up and recom- 

 bined so as to produce secondary changes with new kinds of balance. 

 Many of these new combinations fail to work. The gametes and 

 zygotes carrying them die and fertility is reduced. 



We have also seen how structural hybridity will reduce chromo- 

 some pairing, crossing-over and recombination, or even abolish them 

 altogether, and in so doing remedy the infertility of a polyploid on 

 the one hand and of a gene hybrid on the other. Hence all three 

 types of change, genie, structural and numerical are concerned with 

 one another in permitting the principle of variation to operate in a 

 system of heredity. 



REFERENCES 



CHUKSANOVA, N. 1939. Karyotypes of pollen grains in triploid Crcpis capilUnis. 



Coup. Ranis. [Dokhidy) Acad. Sci. U.S.S.R., 25: 232-235. 

 DARLINGTON, c. D. 1936. The limitation of crossing-over in Oenothera. J. Genet., 



32: 343-352. 

 DARLINGTON, c. D. 1940. The prime variables of meiosis. Biol. Rev., 



15: 307-322. 

 DARLINGTON, c. D. and GALRDNER, A. E. 193?. The Variation system in 



Campanula. J. Genet., 35: 97-128. 

 DARLINGTON, c. D., and MATHER, K. 1944. Cliromosome balance and interaction 



in Hyacinthus. J. Genet., 46: 52-61. 

 KARPECHEKKO, c. D. 1927. Polyploid hybrids of Raphaniis scitivus L. X Brasska 



oleracea L. B. Ap. Botany, 17, 305-408. 



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