446 



Haploids and Autopolyploids 



figuration will open out into a chain of four instead of a ring of 

 four. Pairing sometimes is so arranged that a ring bivalent is 

 produced with one chromosome tied in at each side, as in Fig. 

 1265. If one of the rod chromosomes fails to be tied in by a 

 chiasma, the figure will be a ring and rod trivalent plus a uni- 

 valent. If only two chiasmata form among the arms of the four 



A 



UN 





IQ 



Fig. 126. Some of the configurations formed in a tetraploid when there are 

 four chiasmata. Complete terminalization is assumed. The chromosomes are 

 shown at diplotene and (below) at the subsequent metaphase. Top line 

 from left to right (a), (b), and (c) ; bottom line, (e), (/), (g). 



homologous chromosomes, they may form a chain of three and a 

 univalent (Fig. 127c), two rod bivalents (Fig. 1276), or a ring 

 bivalent with two univalents (Fig. 127a). Other configurations 

 are also possible with two, three, or four chiasmata. If no 

 chiasmata form, which is highly unlikely, four univalents result; 

 and if only one chiasma forms, the only possible configuration 

 is a rod bivalent and two univalents. 



In many autotetraploids pairing relationships result in sev- 

 eral types of configurations in one cell (Fig. 128). Thus there 

 may be two or more types of quadrivalents along with bivalents, 

 univalents, and perhaps a trivalent. Trivalents are rare in pure 



