192 THE BEHAVIOUR OF POLYPLOIDS 



number is small will the triploid produce a moderate proportion of 

 gametes having all the extra chromosomes, owing to their being 

 distributed at random at meiosis (Ch. VIII). Further, of the diploid 

 gametes most are evidently ehminatcd ; since they should combine 

 as frequently as haploids, \'et only one tetraploid appeared in a 

 family along with 59 diploids (Table 49). 



In type E, tetraploid and diploid species are crossed, and, side by 

 side with the expected triploid seedlings, tetraploids are found, 

 differing from them in the preponderant influence of one parent and 

 although they are not simple allotetraploids, in greater fertility. 

 In the Ruhtis hybrid some quadrivalents are formed, as they were 

 in its tetraploid parent, and genetic results show that the pairing in 

 some types of chromosomes is between those derived from the same 

 parent, and in others at random among the four homologues. 



In crosses between races or species with different chromosome 

 numbers we often find that exceptional unreduced gametes are 

 favoured at the expense of reduced gametes. Thus in the following 

 crosses the normally reduced gametes on the diploid side rarely 

 function, and tetraploid seedlings may outnumber triploids : — 



Primula sinensis (^x) x P. sinensis (2x). D., 1931 a. 

 Campanula persicifolia {2%) X C. persicifolia {^x). Gairdner 

 and D., 1931). 



Similarly the cross of diploid Chrysanthemum Makinoi with a high 

 polyploid species succeeded only with the polyploids as male 

 parents and in every case with tetraploid egg-cells produced by 

 failure of both meiotic divisions in the diploid female parent (Table 

 26). 



The greater success of the unreduced or even doubled gametes 

 than the haploid in some circumstances has an important bearing 

 on interspecific crossing. Diploid Brassica oleracea (n = 9) is 

 useless in crossing with B. chinensis (n = 10) and B. carinata, but 

 tetraploid B. oleracea gives highly satisfactory results. Thus 

 doubling of the chromosome number makes possible interspecific 

 crosses that would otherwise fail (Karpechenko, 1935). 



In polyploid crosses a few cases of unexpected and so far in- 

 explicable chromosome distribution have been found (Table 27). 



