iv PRESENCE & ABSENCE THEORY 37 



portions are united anteriorly, but diverge from one 

 another towards the back of the head (Fig. 6, C). 

 The Breda contains an element of duplicity which 

 is dominant to the simplicity of the ordinary single 

 comb. But it cannot contain the factor for the 

 single comb, because as soon as that is put into it 

 by crossing with a single the comb 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 





Rose X Breda 



Duplex v Duplex 



Rose 



Rose 



i I 



Duplex Rose Duplex Single Breda 

 Rose Single (Duplex 



and Simplex) 



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 genera- 

 tion consisting of Bredas, duplex roses, roses, duplex 

 singles, and singles. From our previous experiment 

 we know that the singles 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 

 modifying factor (A*) whose presence turns it into 



