MENDEL'S FIEST LAW 33 



wild type. The abnormal type is treated as the dominant 

 although only when the conditions are favorable to its 

 appearance is the hereditary phenomenon seen. In 

 another case (duplicate legs) only the homozygous form 

 may show the duplications (in a special environment). 

 The following scheme (Fig. 12) represents this relation, 

 reduplication of legs being treated as a recessive : 



Fig. 12. — Relation of normal to duplicate legs. 



There are still other relations that affect the dominance 

 of characters. For example, there may be internal fac- 

 tors, which when present, determine that a character shall 

 be dominant over its allelomorph, or recessive to it. In 

 this connection might be mentioned what has been called 

 "reversal of dominance." An example from Davenport 

 will illustrate what is meant. In a certain strain of fowls 

 there is a tendency for the toes to be united by a web at 

 the base. Crossed to birds with normal feet, no birds 

 with united toes (syndactyls) appeared in F^. The Fj 

 birds inbred gave in F2 only about 10 per cent, of syndactyl 

 birds. It would appear that the latter character is reces- 

 sive, and that the recessive type overlaps largely the 

 dominant heterozygous type. 



Davenport interpreted, however, the syndactyl as the 

 dominant type, because "two syndactyls may give nor- 

 mals, but no true normals give syndactyls." In other 

 words, he defines the dominant type as the one that can 

 carry the other type, because he says dominance is due to 

 presence of factors, recessiveness to absence. "Now 

 dominance may fail to develop but recessiveness never 

 can do so." For this reason two syndactyls may give 



