DIFFERENCES BETWEEN CHROMOSOMES 167 



does not therefore account for metaphase pairing in haploids. 

 From these two circumstances the structurally determined failure 

 of pairing in interspecific hybrids is to be derived. 



Slimming tip : The foregoing observations show three general 

 conditions of the pairing of chromosomes at metaphase of the first 

 meiotic division in diploids, as follows : — 



(i) The presence of similar pairs of chromosomes. 



(ii) Non-di\dsion of the chromosomes permitting their pairing at 

 pachytene. 



(iii) The formation of chiasmata in the paired pachytene chromo- 

 somes. 



In polyploids the pairing of a particular two similar chromosomes 

 depends on a combination of the chance of assortment of them, 

 determined by the number of pairing blocks of which they consist, 

 and the chance of chiasma formation determined by the average 

 chiasma frequency and the interference value. 



Two questions then arise : [a) What kinds of dissimilarity 

 inhibit or restrict chromosome pairing at pachytene ? (b) Wliat 

 relation has metaphase pairing (which can alone be considered in 

 detail) to the degree of dissimilarity occurring ? 



(a) There is evidence of two kinds of dissimilarity or 

 differentiation in the chromatin material. First, there is a 

 qualitative differentiation between particles or groups of particles. 

 This is most simply supposed to be due to differences between their 

 characteristic molecules. A second symptom of these differences is 

 the capacity for specific variation, the property of particles at each 

 locus in a chromosome giving rise to differences (mutations) of a 

 specific character, which has led to such an identifiable particle 

 being described as a " gene." It is probably through gene muta- 

 tions that the qualitative (intramolecular) differentiation arises. 

 Secondly, there are differences due to change in arrangement or 

 structure of these specific particles ; these intermolecular changes 

 determine genetic differences when they lead to changes in quantity, 

 proportion and position. 



Since intragenic changes affect single genes and intergenic 

 changes affect larger or smaller groups of genes (perhaps many 

 hundreds) , though both may inhibit pairing, on the simple assump- 



