172 GENETICS 



When the food becomes dried up and there is a mini- 

 mum of moisture the banding on the abdomen disap- 

 pears. Here is a reversal of dominance but the gene 

 itself is not affected since the same flies which are 

 hybrid with respect to the character of banding, show 

 a difference according to the environment of food and 

 moisture, in the amount of banding given expression. 

 Notched margin in leaves is dominant in nettles and 

 recessive in the celandine. Again a negative character 

 may be the dominant one in a pair of allelomorphs. 

 For example, the bob-tail of the Manx cat is dominant 

 over the ordinary long tail of the cat; the reduced 

 number of three digits in guinea-pigs is dominant over 

 four digits ; the polled condition is dominant over horns 

 in cattle; the rumpless fowl is dominant over the fowl 

 with a rump, and brachydactyly in man, that is, fingers 

 or toes with only two joints each, is dominant over 

 the three- jointed arrangement. 



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 



