FAILURE OF PAIRING 165 



parts in which the chiasmata are formed, and trivalents in a triploid 

 will be less frequent if chiasmata are localised. This has not yet 

 been proved. Secondly, proportionate dissimilarities will reduce 

 pairing more considerably in a type with low than in one with high 

 chiasma frequency, a respect in which clones of Fritillaria 

 imperialis differ [cf. Ch. VII), and in short chromosomes more than in 

 long ones. Thirdly, where pairing fails as a result of structural 

 hybridity it will vary in the extent of its failure in different nuclei, 

 following a unimodal curve such as that found in trivalent formation 

 and in chiasma frequency itself. It will also show variation in 

 different individuals from other conditions affecting chiasma 

 frequency [cf. Ch. VII and Fig. 57). 



(/) The occurrence of interlocking and its origin as shown by 

 Gelei {v. Ch. VII) and the occurrence of exchanges of partner at 

 zygotene in polyploids (D., 1929 c) indicate that the accidents of 

 relative position may reduce the completeness of pairing. These 

 observations apply equally to pairing in translocation-heterozygotes, 

 for the same exchanges of partner occur optionally amongst the 

 homologous chromatids of non-hybrid polyploids as occur necessarily 

 amongst the interchanged chromosomes of the heterozygotes. It 

 is therefore not surprising to find in certain polyploids a considerable 

 reduction of chiasmata, and, with terminalisation, many more free 

 ends than in the corresponding diploids. This is increasingly 

 evident in the series : Datura Stramonium (Belling, 1927), Primula 

 sinensis (D., 1931 a), Tradescantia virginiana (D., 1929c), Campanula 

 persicifolia (Gairdner and D., 1931). 



Interference of one chromosome with another at zygotene is 

 greater in its effect on the longer chromosomes. Thus in triploid 

 Hyacinthus the long chromosome type has a much wider variation 

 in the length paired at zygotene than the short and medium 

 chromosomes. It pairs as though it consisted of only two blocks 

 which pair, or do not pair, at random with regard to one another. 

 It therefore has a higher proportion of unpaired chromosomes at 

 metaphase than the medium type, in spite of its higher chiasma 

 frequency (v. Ch. IV). 



In hybrids with some polarisation of the nucleus at zygotene, 

 interference with pairing from structural hybridity will give an 



