206 



THE BEHAVIOUR OF POLYPLOIDS 



Pachy^cr^c Diplo^er^e 



M 



Triplci(iy 



Fig. 73. — Diagram showing the method of pairing in an allotetraploid 

 species and in the " haploid " and " triploid " derived from it on 

 the assumption that the dissimilar chromosomes are similar and 

 capable of pairing, in the absence of competition, in half their 

 length. M, metaphase ; black, similar segments ; white and 

 hatched, dissimilar ones. Note : pairing of dissimilar chromo- 

 somes is inevitable in " haploid " and " triploid " but only possible 

 in the face of competition in the species, whence differential 

 affinity. (i) and (ii) are alternative possibilities with and 

 without autosyndesis. 



The conditions determining these results may be explained on 

 the assumptions (i) that the differences which reduce pairing at 



