THE CENt AS A UNIT OF CHANGE 

 TABLE 11 



FOUR ALLELOMORPHS GOVERNING SIZE OF EYE IN 

 FLOWERS OF PRIMULA SINENSIS 



Two interpretations are clearly possible. Either (i) a has two effects 

 independent from the beginning and one of them distinct from that 

 shown by any of the others, even the otherwise indistinguishable a". 

 This means two units of action, separable in their changes, but 

 inseparable in recombination, so far as our experience goes. For 

 if short-style could occur with normal eye we have not yet recog- 

 nized it. Or (ii) a" does the same thing as a but does it more 

 effectively, thereby passing a threshold which introduces a new or 

 secondary ultimate effect, the visible change in style length. This 

 means one unit of action as well as of recombination, but supposes 

 a relation in development between eye and style of which we other- 

 wise know nothing. The distinction betw^een these alternatives is 

 thus as follows: 



Complete Linkage 

 Two genes having distinct 

 initial effects but inseparable 

 by recombination. 



Pleiotropic Action 

 One gene having a single 

 initial effect with manifold 

 expression. 



The choice between these alternatives is important wherever 

 manifold expression is concerned, whether multiple allelomorphs 

 are present or not. In every case it must depend on our knowledge 

 of the whole series of steps from the gene to its expression. This 

 knowledge is perhaps available in certain simple cases. In other more 

 complicated cases we must turn to new ways of attacking the prob- 

 lem. If we can show that the series of steps that we do know can 

 be traced back to one unit action, a single gene of manifold effect 

 must be at work. 



The tracing of gene action has been done in two ways. Embryo- 



115 



