180 GENETICS 



by a duplex dose of the same character. It is as if 

 a single bottle of black ink poured into a jar of water 

 was just as effective as two bottles of ink, in forming 

 an opaque fluid. 



b. Partial Potency 



Partial potency covers all cases of incomplete 

 dominance, such as those of the four-o'clock (Mirabilis) 

 and blue Andalusian fowls, where a simplex dose of 

 a determiner does not produce the same visible effect 

 as a double dose. 



The dominant prickly Jamestown weed (Datura), 

 when crossed with a recessive glabrous variety of 

 the same plant, produces cross-breds in the first 

 generation which show only a few prickles (Bateson) 

 (Baur), following the law of partial potency. 



Banded and uniformly colored snails also, when 

 crossed together, produce snails with shells showing 

 only a pale banding (Lang). 



Numerous further instances of incomplete domi- 

 nance could be cited. 



c. Failure of Potency 



If for any reason a determiner fails to accom- 

 plish its possibilities in whole or in part, then the 

 character in question may never become evident, and 

 the result, so far as appearances go, is the same as 

 if it was a recessive lacking the determiner entirely. 



That the failure of potency is not identical with 

 the absence of a determiner can usually be demon- 

 strated by further breeding, because dominants failing 



