THE INBREEDING HYBRID 



heterozygous Oenothera is selfed, however, it does not produce a 

 mendchan combination of i : 2 : i. The two classes of homozygotes 

 fail to appear. The hybrid breeds true. Indeed it is only when a 

 heterozygous species is crossed with another, heterozygous or 

 homozygous, that its heterozygosity is shown by the mixture in the 

 progeny, by the production of twin hybrids. Thus if the second 

 species is homozygous for a set y with the pairs: AB CE DF 



AB CE DF 

 then the crossed seedlings will be of two kinds : — 



AB CE FD \y AB CE DF 



\ / \ / and \ \ /\/\/ 



AB EF DC U BC ED FA 



These two kinds will be recognizable by the differences in the size 

 of the ring and in their external appearance. 



These observations present us with two problems. Why do sets 

 or complexes, with different arrangements of chromosomes pro- 

 duced by interchange, differ in their phenotypic effects ? And why 

 do their homozygous combinations so often fail to appear, or at 

 least to live? Renner showed that sometimes, as we have already 

 seen, one complex fails in the pollen and the other in the egg. 

 He also showed that, in many other cases, homozygotes were often 

 formed, but quickly died. Evidently each set is defective in some 

 way or other, and its defect is covered by its partner. If we can 

 find out what this defect is we shall have explained why the different 

 sets have different effects. 



A moment's consideration will show that our symbols AB, BC 

 and so on make an unjustifiable assumption. Chromosomes, as we 

 saw, can and do break and rejoin at various points, indeed between 

 any two genes. They are not each composed of two unbreakable 

 arms exchangeable only at the centromere. There are segments 

 therefore (as we saw earlier in Fig. 32) which are omitted from 

 our diagram between A and B, between B and C, and so on. The 

 sum of these segments in one complex would be equal originally 

 to the sum of those in its partner complex, provided that no super- 

 fluous breaks or mutations had ever occurred. But it is of the nature 

 of chromosomes that superfluous breaks and mutations do occur. 

 When they occur in the complex heterozygote their consequences 



261 



