70 MENDELISM chap. 



work of Biffen showed that the beardless behaved as 

 a simple dominant to the bearded condition with its 

 long awns (Fig. 14). The beard is obviously the 

 additional character, and on the Presence and Absence 

 theory we should have expected it to be dominant. 

 The fact that it was not so suggested that the beard- 

 less wheats with which Biffen worked, contained an 

 inhibitor suppressing the growth of the beard. This 

 suggestion has recently been confirmed by Howard, 

 who has been able to demonstrate two forms of 

 beardless wheat, one of which is dominant and the 

 other recessive to the bearded form. When these 

 two kinds of beardless wheats are crossed, they 

 produce a certain proportion of bearded offspring in 

 Fj, just as the two kinds of white poultry produce a 

 certain number of coloured birds. 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 omis- 

 sion of a factor. The dominant and recessive white 

 poultry are indistinguishable 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^. But in other families 

 similarly bred the proportions of birds with and with- 

 out the extra toe appear to be unusual. It has 



