BASES OF CHANCE 



Misdivision of the Centromere 



There is one change in the hereditary mechanism, and one only, 

 which affects both the number and the structure of the chromosomes 

 at a single stroke. That change is the misdivision of the centromere. 



The unpaired chromosome at the first anaphase of mciosis is in 

 an equivocal position. Its centromere is unable to co-orientate w^ith, 

 or segregate from, a partner. Nor can it divide at the same time as 

 the paired chromosomes segregate. It is therefore frequently lost. 

 Sometimes, however, its difficulty is resolved by division within 

 itself. Instead of separating lengthwise from the product of its own 

 reproduction (which is not yet available) it separates crosswise into 

 two components. It thus proves to be a compound or repetitive 

 gene. Sometimes it explodes to give useless fragments, but often it 

 divides into what must be two nearly equal halves, for each part 

 successfully carries its chromosome arm to the pole and undergoes 

 a second division of its new chromosome. 



The new chromosome is telocentric and in some circumstances is 

 stable. Both in Orthoptera and in the plant Campanula persicifolia 

 races have been found where a particular chromosome is replaced 

 by its two telocentric arms so as to add one more to the haploid 

 number. But this is not always so. At the next mitosis (in the pollen 

 grain ofFritillaria for example) reunion of the two daughter chroma- 

 tids is found to have taken place within the centromere. The new 

 chromosome, which passes to one pole without division, is then twice 

 the size of the telocentric. It has a double-sized centromere and two 

 identical arms. It is known as an iso-cliromosomc. A pair of iso- 

 chomosomes are in effect tetrasomic. They have been found as part 

 of the regular complement in Nicatnlra and as supernumeraries 

 varying in number in different plants in Sorghttm, Sccale, and Datura. 



Thus the primary changes of inversion, interchange, translocation 

 and misdivision can never change the balance. But by recombination 

 secondary changes occur, producing a new content for the nucleus 

 as a whole. 



Deficiency and Balance 



Deficiencies in parts o{ single chromosomes, if they arc small 

 enough, are not always fatal to diploid nuclei. But when such a 



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