THE CELL-DOCTRINE 137 



shown in Fig. 18. The Fi individuals present the same ap- 

 pearance as the dominant parent. They are without excep- 

 tion gray, as though they were pure gray mice. The reces- 

 sive feature is as though absent. But these Fi mice cannot 

 be exactly like the pure grays of the Pi, because they had 

 one white parent. There must, therefore, be two kinds of 

 grays, those which have two pure gray parents and those 

 which have one pure gray and one white parent. The ex- 

 istence of two kinds of grays is proved by the subsequent 

 breeding. In the F 2 generation the white reappears. The 

 ratio is 75% gray to 25% white, as shown by the figure. 

 Study of these F 2 individuals solves the problem. The 

 1:2:1 ratio is present, only it is masked by the dominance 

 of gray over white. The 75% of gray mice is shown by breed- 

 ing tests to consist of 25% pure gray like the Pi grandparent, 

 plus a 50% of hybrid grays like the Fi generation. The real 

 ratio is: 25% Pure Dominants: 50% Hybrid Dominants: 

 25% Recessives. The recessives are always distinguish- 

 able, since they are what they seem to be on the outside. 

 The dominants are of two types pure and hybrid. Using 

 symbols, the formula 1 DD : 2 DR : 1 RR, or 1 GG : 

 2 GW : 1 WW expresses the facts. It happened that 

 dominance appeared in the cases originally described by 

 Mendel. Its importance was consequently overrated. 

 Many instances are now known in which dominance is 

 incomplete and others in which the terms dominant and 



in particular cases. Dominance may be incomplete, in which event the hybrid 

 can be at once distinguished from the dominant parent. Where dominance is 

 complete the hybrid and the dominant parent are indistinguishable. In many 

 cases the dominant character is observed to be the presence of some quality 

 and the recessive character the absence of the same. Thus the gray mouse has 

 pigment present in its coat, the white mouse has pigment absent. The normal 

 human eye has pigment present, the albino eye is devoid of this characteristic, 

 so that albino eyes are red being colored only by the blood. These facts have 

 led to the presence-and-absence theory, which explains dominance in the manner 

 indicated. Incomplete dominance in the Fi generation might be described by 

 saying that often a single dose of the character is not sufficient to give complete 

 resemblance to the dominant parent. 



