BLENDING INHERITANCE 175 



by further breeding, because dominants failing in po- 

 tency, which are either of the formula DD or DR, 

 may, if bred inter se, give a various progeny among 

 which the dominant character D is likely to become 

 manifest again, while recessives, of the formula RR, 

 on the contrary, will invariably give offspring which 

 all agree in the entire absence of the character in 

 question. 



Davenport cites an extreme case of failure of po- 

 tency in one of two rumpless cocks from the same blood. 

 The character of rumplessness is due to an inhibitor 

 of tail development. That these two cocks both pos- 

 sessed this character was demonstrated by the entire 

 absence of any tail in either case. The inhibiting de- 

 terminer for tail growth was so weak in cock No. 117, 

 however, that, to quote Davenport's exact words: "In 

 the heterozygote the development of the tail is not 

 interfered with at all, and even in extracted dominants 

 it interfered little with tail development, so that it 

 makes itself felt only in the reduced size of the uro- 

 pygium and in-bent or shortened back. But in No. 116 

 the inhibiting determiner is strong. It develops fully 

 in about 47 per cent of all the heterozygotes and in 

 extracted dominants may produce a family in all of 

 which the tail's development is inhibited." 



Here were two birds of the same blood, phenotyp- 

 ically alike and presumably genotypically alike, which 

 because of an individual difference in the potency of 

 the determiner for rumplessness produced quite differ- 

 ent results in their offspring although bred to precisely 

 the same array of hens. 



