Nondisjunction 427 



regularly form rings or circles, the size and number of which 

 depend upon the number and position of the translocations for 

 which they are heterozygous. For example, in Oe. Lamarckiana, 

 the chromosomes are arranged at meiosis in the form of one pair 

 and a circle of twelve. The pair represents a chromosome that 

 bears no translocation. The circle is composed of one set of 

 chromosomes whose ends are designated as 3-4, 5-8, 7-6, 9-10, 

 11-12, and 13-14 and one set whose ends are 3-14, 5-6, 7-4, 

 12-10, 11-8, and 13-9. Terminalization is complete in Oenothera, 



velana 



(l 21 I l3 4| 17 el 15 81 111 I2I 110 9I 113 141 I 



U\ I 14 71 16 51 18 111 112 101 19 131 114 31 



'' \j \j \j Kj \j KJ 



gaudens 



Fig. 118. Arrangement of the chromosomes at the first meiotic meta- 

 phase of Oenothera Lamarckiana showing one pair and a circle of twelve 

 chromosomes. Six of the chromosomes belong to the velans complex and 

 normally always go to one pole; the other six belong to the gaudens com- 

 plex and go to the opposite pole. These two complexes have arisen as the 

 result of a number of reciprocal translocations. 



and normally the chromosomes are arranged so that alternate 

 ones go to the same pole, and therefore always remain together. 

 Such a group of chromosomes is called a complex; and to facili- 

 tate discussion about the various interchange types, the dif- 

 ferent complexes are given names. Thus the first group of chro- 

 mosomes above, along with one of the 1-2 chromosomes, is called 

 the velans complex; the second group with one of the 1-2 chromo- 

 somes is the gaudens complex. Because of the position and num- 

 ber of the interchanges between these two complexes, they form 

 one pair and a circle of twelve when they are together in the 

 same plant. 



The chromosomes are so oriented that the ones of the velans 

 complex go to one pole while the gaudens chromosomes go to the 

 other (Fig. 118). Apparently both the velans and gaudens com- 

 plexes carry lethal genes or small deficiencies which are dif- 

 ferent in the chromosomes of the two complexes. Therefore, 

 both velans • velans and gaudens • gaudens forms are homozy- 



