+------+------+------+------+ |CI |CI |CI |CI | |CI |Ci |cI |ci | | | | | | | | | | | +------+------+------+------+ |Ci |Ci....|Ci |Ci....| |CI |Ci....|cI |ci....| | |......| |......| | |......| |......| +------+------+------+------+ |cI |cI |cI |cI | |CI |Ci |cI |ci | | | | | | | | | | | +------+------+------+------+ |ci |ci....|ci |ci | |CI |Ci....|cI |ci | | |......| | | | |......| | | +------+------+------+------+ FIG. 13. Diagram to illustrate the nature of the F_2 generation from the cross between dominant white and recessive white fowls. 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 compared 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 {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 development of colour. If we denote this factor by C and our postulated inhibitor factor in the dominant white bird by I, 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 constitution of the F_1 bird must be CcIi. Such birds being heterozygous for the inhibitor factor, should be whites showing some coloured "ticks." Being heterozygous for both of the two factors C and I, they will produce in equal numbers the four different sorts of gametes CI, 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 I; these will be white birds either with or without a few coloured ticks. Three contain C but not I: these must be coloured birds. One contains neither C nor I; 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 {74} deduced were found to accord with the actual facts of experiment.