120 THE AMERICAN NATURALIST [Vol.XLVI 



X absence of red gives the usual Mendelian phenomena of 3 red 

 cob: 1 white in F 2 . Likewise, the cross red pericarp (with white 

 cob) X absence of red gives 3 red pericarp : 1 white. Thus each 

 of these types (factors) behaves as an allelomorph to its absence. 

 Hence they should be due to genetically independent genes. But 

 such is not the case ; they are allelomorphic to each other. 



Some students would say that this is because they repel each 

 other. But this explanation does not satisfy in this case, for it 

 can hardly be doubted that red cob in the one ease and red peri- 

 carp in the other are due to the same cause, acting differently 

 in the two cases. I am of opinion that many similar cases of 

 factors behaving as allelomorphs to their absence will be found 

 to be also allelomorphic to each other. Such cases have usually 

 not been looked for. Quite a number of them have been reported, 

 and I hope some time to be able to bring them all together for 

 reference. Emerson 7 gives a case in beans, in which a variety 

 with green leaves and green pods was crossed with another hav- 

 ing green leaves and yellow pods. F 2 consisted of three of the 

 former to one of the latter. Here green was allelomorphic to 

 absence of green. Later he crossed two varieties, one with green 

 leaves and green pods, the other yellow leaves and pods. F 2 con- 

 sisted of three of the former and one of the latter. Here green 

 was again allelomorphic to its absence. These two crosses ap- 

 parently show that yellow pods with green leaves and yellow 

 pods with yellow leaves are not genetically distinct. Yet if yel- 

 low had been dominant in both these cases, it would have been 

 the usual custom to consider that the two different yellows were 

 independent genetically, because each was allelomorphic to its 

 absence. It would be interesting to know how the two yellows 

 would behave if crossed. It would not be at all surprising to 

 find these two "absences" exhibiting the phenomenon of spur- 

 ious allelomorphism. The case would be still more interesting if 

 a variety could be found with yellow leaves and green pods. 



W. J. Spillman 



(To be continued) 



