iv PRESENCE AND ABSENCE THEORY 41 



assumes a large size, and is totally distinct in appearance 

 from its almost complete absence in the pure Breda. 

 Now when the Breda is crossed with the rose duplicity is 

 dominant to simplicity, and rose is dominant to lack of 

 comb, and the F 1 generation consists of birds possessing 

 duplex rose combs (Fig. 6, A and B). On breeding such 

 birds together we obtain a generation consisting of 

 Bredas, duplex roses, roses, duplex singles, and singles. 

 From our previous experiment we know that the singles 



Rose X Breda 



I . 



Duplex x Duplex 



Rose 



Rose 



r r 



Duplex Rose Duplex Single Breda 



Rose Single (Duplex 



and Simplex) 



could not have come from the Breda, since a Breda comb 

 to which the factor for single has been added no longer 

 remains a Breda. Therefore it must have come from the 

 rose, thus confirming our view that the rose is in reality a 

 single comb which contains in addition a dominant modi- 

 fying factor (R) whose presence turns it into a rose. We 

 shall take it, therefore, that there is good experimental 

 evidence for the Presence and Absence theory, and we 

 shall express in terms of it the various cases which come 

 up for discussion in succeeding chapters. 



