Translocations 



405 



configuration may more nearly approach a cross than a ring. 

 In organisms in which terminalization is complete or nearly so, 

 the chromosomes tend to become oriented on the metaphase plate 

 in such a way that alternate chromosomes go to the same pole; 

 in other forms, the chromosomes line up in a purely random 

 manner. The ring may consist of more than four chromosomes 



a o c 



Fig. 109. Pairing of chromosomes in an individual heterozygous for a 

 reciprocal translocation, (a) The two original chromosomes (left) and the 

 two interchange chromosomes. (6) Diplotene showing eight chromatids 

 and one or two chiasmata in each arm. (c) The following metaphase (not 

 showing chromatids) assuming complete terminalization of chiasmata. The 

 chromosome at the left in (a) would be chromosome ab cd oi the text and 

 the one immediately to the right chromosome k I m. Next in order would 

 be the ab m and klcd chromosomes mentioned in the text. 



as in numerous wild strains of Oenothera and in Rhoeo (Fig. 

 110), and such large rings may also be produced by X-rays. 



In plants like Datura and Oenothera, where there is a consid- 

 erable number of reciprocal translocations in various strains 

 but in which terminalization is complete, the rings are usually 

 oriented as in Fig. Ilia, with alternate chromosomes going to 

 each pole and therefore with alternate chromosomes included in 

 each type of gamete. In plants in which there is no terminaliza- 

 tion, however, all three possible orientations (Fig Ilia, h, and c) 

 are usually found with approximately equal frequency. If, in a 

 given plant or animal, the passing of alternate chromosomes to 

 the same pole is a regular feature of meiosis, all the spores or 

 gametes of that organism will be viable and functional and that 



