vii DOMINANCE 73 



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 development 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 

 recessive to colour, and that the whiteness of such birds 

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

 ment of colour. If we denote this factor by C and our 

 postulated inhibitor factor in the dominant white bird by 

 /, then we must write the constitution of the recessive 

 white as ccii, and the dominant white as CCII. We may 

 now work out the results we ought to obtain when a cross 

 is made between these two pure white breeds. The con- 

 stitution of the FI bird must be Ccii. Such birds being 

 heterozygous for the inhibitor factor, should be whites 

 showing some coloured " ticks.' 7 Being heterozygous for 

 both of the two factors C and /, they will produce in equal 

 numbers the four different sorts of gametes C/, Ci, ci, ci. 

 The result of bringing two such similar series of gametes 

 together is shown in Fig. 13. Out of the sixteen squares, 

 twelve contain / ; these will be white birds either with or 

 without a few coloured ticks. Three contain C but not / : 

 these must be coloured birds. One contains neither C 

 nor / ; this must be a white. From such a mating 

 we ought, therefore, to obtain both white and coloured 

 birds in the ratio 13 : 3. The results thus theoretically 



