70 MENDELISM CHAP. 



case, for instance, is that of bearded and beardless 

 wheats. Though the beard is obviously the addi- 

 tional character, the bearded condition is recessive to 

 the beardless. Probably we ought to regard the 

 beardless as a bearded wheat in which there is an 

 inhibitor that stops the beard from growing. It is 

 not unlikely that as time goes on we shall find 

 many more such cases of the action of inhibitor 

 factors, and we must be prepared to find that the 

 same visible effect may be produced either by 

 the addition or by the omission of a factor. The 

 dominant and recessive white poultry are indistin- 

 guishable in appearance. Yet the one contains a 

 factor more and the other a factor less than the 

 coloured bird. 



A phenomenon sometimes termed irregularity of 

 dominance has been investigated in a few cases. 

 In certain breeds of poultry such as Dorkings there 

 occurs an extra toe directed backwards like the hallux 

 (cf. Fig. 15). In some families this character behaves 

 as an ordinary dominant to the normal, giving the 

 expected 3 : I ratio in F 2 . But in other families 

 similarly bred the proportions of birds with and with- 

 out the extra toe appear to be unusual. It has 

 been shown that in such a family some of the birds 

 without the extra toe may nevertheless transmit the 

 peculiarity when mated with birds belonging to 

 strains in which the extra toe never occurs. Though 

 the external appearance of the bird generally affords 

 some indication of the nature of the gametes which 

 it is carrying, this is not always the case. Nevertheless 

 we have reason to suppose that the character segregates 

 in the gametes, though the nature of these cannot 



