260 



AN INTRODUCTION TO MODERN GENETICS 



chromosome. Some of the extra Vulgare 7 chromosomes, which we 

 may call ahcdefg^ may be present in addition. The phenomenon of the 

 increasing and decreasing groups seems to be due to the fact that too 



Fig. 114. Chromosome Behaviour in the F1 Hybrid between Tetraploid 

 and Hexaploid Wheats. — A is a polar view of the metaphase of the first division, 

 showing 14 bivalents and 7 univalents. In the anaphase B the univalents lag behind 

 on the spindle, and eventually split and the chromatids separate to the two poles. 

 In the second division anaphase C the single chromatids are separated at random 

 to the two poles. 



(After Sax.) 



great an unbalance produces sterility. Tetraploids are fertile if they 

 have any number of single extra chromosomes (e.g. 14 Emmer + 14 

 vulgare + ahcef) but not if any chromosome is represented twice 

 (e.g. 14 E. + 14 ^. + abccd); this leads to the loss of extra chromo- 



Fig. 115. Spartina Townsendii. — Mitotic metaphase plates of S. Townsendii (A) 

 and its probable ancestors S. alterniflora (6) and S. stricta (C). 



(After Huskins.) 



somes and the resumption of the tetraploid number by the diminishing 

 group. On the other hand, pentaploids with extra single chromosomes 

 are also fertile (e.g. 14 E -{- 14 z^ + abcdefg + abc) and these will 

 gradually revert to the hexaploid number. 



There are several other grasses which are related to the wheat poly- 

 ploids, the closest relative probably being Aegilops. Some fertile crosses 



