158 Discussion of Coupling [CH. 



The time has not yet come for such an analysis to be 

 attempted. Nevertheless we can scarcely forbear from con- 

 sidering some of the possibilities which suggest themselves. 



In spurious allelomorphism the outward facts are com- 

 paratively simple. Two dominant, or " present " factors, 

 behave as if in the cell-divisions of gametogenesis they 

 repelled each other, and we must suppose that this repulsion 

 is exerted at some definite cell-division, such that one factor 

 passes into one daughter-cell and the other factor into the 

 other. The dividing cell being AaBb, the daughter-cells 

 are respectively Ab and aB. Though as yet only one case 

 has been definitely proved to follow this system, the 

 evidence in that case is very positive. Moreover when the 

 facts of sexual inheritance come to be related, a group of 

 cases will be described which conform so precisely with this 

 type-example of spurious allelomorphism that it is practically 

 certain that this case is not a solitary example, but one 

 which typifies a category of genetic phenomena. It may 

 therefore be tak^n that repulsion or, more strictly, a 

 relation which can be represented as repulsion may exist 

 between factors belonging to distinct allelomorphic pairs. 



The state of things which results in gametic coupling 

 is much more obscure. The association of characters here 

 is quite distinct from the association of characters produced 

 by spurious allelomorphism. In gametic coupling the 

 dominant factors are associated together, while in spurious 

 allelomorphism the dominant factors are dissociated from 

 each other. If the coupling were total, so that all the 

 gametes were either AB or ab, just as in spurious allelo- 

 morphism they are all either Ab or aB, we might naturally 

 suppose the one phenomenon to be the converse of the 

 other. The one might then be represented as an effect of 

 attraction just as the other may be represented as the 

 result of repulsion between the two dominant factors. So 

 far, however, as experiment has yet gone, we have no 

 certain case in which the coupling is complete. There are 

 no doubt instances of features apparently distinct which 

 are nevertheless transmitted in collocation. In the Sweet 

 Pea, for instance, the deep brown or blackish pigmentation 

 of the seed-coat occurs only in plants with some colour in 

 the flower, but these two features may thus be supposed to 



