76 MENDELISM chap. 



A phenomenon sometimes termed irregularity of domi- 

 nance 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 F2. But in other families similarly bred the pro- 

 portions of birds with and without 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 be- 

 longing 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 can- 

 not always be decided from the appearance of the bird 

 which bears them. 



There are cases in which an apparent irregularity of 

 dominance has been shown to depend upon another 

 character, as in the experiments With sheep carried out by 

 Professor Wood. In these experiments two breeds were 

 crossed, of which one, the Dorset, is horned in both sexes, 

 while the other, the Suffolk, is without horns in either 

 sex. Whichever way the cross was made the resulting 

 Fi generation was similar; the rams were horned, and 



