vii DOMINANCE 67 



a visible distinction between the heterozygote and 

 the pure dominant. There are certain white breeds 

 of poultry, notably the White Leghorn, in which the 

 white behaves as a dominant to colour. But the 

 heterozygous whites made by crossing the dominant 

 white birds with a pure coloured form (such as the 

 Brown Leghorn) almost invariably show a few 

 coloured feathers or " ticks " in their plumage. The 

 dominance of white is not quite complete, and renders 

 it possible to distinguish the pure from the impure 

 dominant without recourse to breeding experiments. 

 This case of the dominant white fowl opens up 

 another interesting problem in connection with 

 dominance. By accepting the Presence and Absence 

 hypothesis we are committed to the view that the 

 dominant form possesses an extra factor as com- 

 pared with the recessive. The natural way of 

 looking at this case of the fowl is to regard white 

 as the absence of colour. But were this so, colour 

 should be dominant to white, which is not the case. 

 We are therefore forced to suppose that the absence 

 of colour in this instance is due to the presence of 

 a factor whose property is to inhibit the production 

 of colour in what would otherwise be a pure coloured 

 bird. On this view the dominant white fowl is a 

 coloured bird plus a factor which inhibits the de- 

 velopment of the colour. The view can be put to 

 the test of experiment. We have already seen that 

 there are other white fowls in which white is reces- 

 sive to colour, and that the whiteness of such birds 

 is due to the fact that they lack a factor for the 

 development of colour. If we denote this factor by 

 C and our postulated inhibitor factor in the dominant 



