126 MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



Where only a single extra chromosome is present in trisomic 

 plants it behaves in the same way as do each of the extra 

 chromosomes in a triploid {e.g., Solanum, Lesley, 1929 ; Matthiola, 

 Philp and Huskins, 1931 ; Datura, Belling, 1927). Tetraploids 

 with complete terminalisation show very clearly the further 

 interesting property that although the chromosomes must be in 



Table 15 



Variation in Pairing of Chromosomes in Triploids and Tetraploids 



(Cf. Table 14.) 



approximately symmetrical associations at pachytene (every change 

 of partner being reciprocal) their metaphase associations occur in 

 all the possible ten types, of which only four are symmetrical 

 (having both ends of all the chromosomes behaving similarly). 

 Association is therefore variable, not only as affecting whole chromo- 

 somes but as affecting their parts. 



Now it has been seen that the chromosomes of bivalents appear 

 to be held together at diakinesis and metaphase merely by chiasmata, 

 for the chromosomes themselves repel one another. And it has 

 been seen also that chiasmata are usually formed in a number 



