48 



INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



are heterozygous they must produce 25 per cent recessives. This is the fact 

 that forces us to conclude that clean shank is not recessive, but dominant 

 and due to an inhibitor that frequently fails to dominate. In table 31 the 

 two recessive varieties, mated inter se, produce no featherless shanks; the 

 feathers grow freely as they do over the rest of the body. Some of the 

 Silkies of table 31, however, are really heterozygous, with the dominant 

 inhibitor not showing; consequently they throw a large proportion of non- 

 booted offspring. In F„ as table 33 shows, the heterozygous offspring have 

 a reduced boot and perfect dominance — complete inhibition of boot — ^in 

 from 6 to 68 per cent. Dominance is most complete in the Silkies, where, 

 the feathering being feeble, the inhibitor has, as it were, less to do in over- 

 coming it. In Fj the expected 75 per cent dominant is approached in the 

 case of the Silkies (62 per cent and 59 per cent, respectively), but inhibition 

 is very imperfect in the Cochin and Brahma crosses, being reduced to between 

 25 and 2 per cent. More proof that boot is due to the absence of a factor 

 rather than to its presence is found in this generation. If absence of boot 

 is recessive, then, combined with imperfection of dominance, at least 25 per 

 cent of the offspring should be recessive and probably a much larger pro- 

 portion. The results in table 34 are absolutely incompatible with this 

 hypothesis, since, in one case, there are only 2 per cent that can not develop 

 boot. Two extracted clean-footed birds sometimes throw boot and some- 

 times not, and this result is to be expected on the hypothesis that clean- 

 footedness is dominant, but two heavily booted birds can not transmit 

 the boot inhibitor. 



Table 34. — Distribution of hoot-grade in the F^ generation of booted X non-booted poultry. 



