BLENDING INHERITANCE 179 



it fails, for some reason, to become effective. This 

 interpretation agrees with the facts brought out by 

 subsequent breeding in cases of this sort. 



It sometimes occurs that a character which is 

 dominant in one species may be recessive in another. 

 Horns are dominant in sheep, but recessive in cattle. 

 White color is recessive in rodents and sheep, but 

 dominant in most poultry and in pigs. 



5. POTENCY 



Davenport seeks to explain modifications in typical 

 dominance as variations in the potency of determiners. 

 He defines potency as follows: "The potency of a 

 character may be defined as the capacity of its germi- 

 nal determiner to complete its entire ontogeny." 



That is, if the potency of a determiner, for some 

 reason, is insufficient, there may be either an incom- 

 plete or delayed manifestation of the character in 

 question, or it may fail entirely to develop. 



The variations of potency may be grouped into 

 three general categories according to the degree of 

 their manifestation ; namely, total potency, partial 

 potency, and failure of potency. 



A further word of explanation for each of these 

 three kinds of potency seems desirable at this point. 



a. Total Potency 



This is complete Mendelian dominance in which 

 even the heterozygotes produced by a simplex dose 

 of a character are indistinguishable phenotypically, 

 that is, by inspection, from the homozygotes produced 



