74 THE THEORY OF THE GENE 



another time, in another bird, a dominant mutation oc- 

 curred that gave a rose comb, it follows that in the cross 

 described above, the F 2 single comb is due to the presence 

 of the original wild type genes. Thus, a race with pea 

 comb (PP) will contain the wild type genes (rr), from 

 which the rose comb arose by mutation. Similarly the 

 race with rose comb (RR) will contain the wild type 

 genes (pp), from which the pea comb arose by mutation. 

 The formula for the pea comb race is then PPrr and 

 for the rose comb race RRpp. The germ cells of these two 

 races will be Pr and Rp respectively, and the F 1 will be 

 PpRr. The two dominants produce a new type, the wal- 

 nut comb. Since two pairs of genes are present in ¥ 1 

 there will be 16 combinations in F 2 , and of these one will 

 be pprr or single comb. The single comb is due then to 

 the recombination of the wild type recessive genes that 

 entered the cross. 



Recessive Characters and Absences of Genes. 



In the background of the presence and absence theory 

 there lurks, beyond doubt, the idea that many recessive 

 characters are actual losses of some character that was 

 once present in the original type, hence by implication the 

 gene of that character is also absent. This idea is a hang- 

 over of Weismann's theory of the relation of determinant 

 to character. 



It is instructive to look a little closer into the evidence 

 that may have seemed at first to support such an inter- 

 pretation. 



An albino rabbit or rat or guinea pig may be inter- 

 preted to have lost the pigment characteristic of the origi- 

 nal type. In a sense no one will deny that the relation of 

 the two types may be expressed in this way, but, in pass- 

 ing, it may be noted that many albino guinea pigs have a 

 few colored hairs on the feet or toes. If the pigment-pro- 



