60 HEREDITY AND EVOLUTION IN PLANTS 



of the hybrid plants are smooth. The inheritance was 

 "smooth" and "wrinkled," but the expression was of 

 only one type smooth. A character thus expressed, to 

 the exclusion of another, in the first filial (Fi) genera- 

 tion Mendel called dominant, and the phenomenon he 

 called dominance; the other character is recessive. From 

 such observations Mendel formulated the law of domi- 

 nance, as follows: When pairs oj contrasting characters 

 are combined in a cross, one character behaves as a dominant 

 over the other, which is recessive. 



By similar experiments Mendel found that, in the coty- 

 ledons, yellow is dominant over green, tallness over dwarf- 

 ness, axial flowers over terminal, and so on. Such pairs 

 of contrasting characters are called allelomorphs. 



2. Law of Segregation. But what will happen if the 

 first filial (Fi) generation is inbred or self-pollinated. Its 

 inheritance included factors that make for both "smooth" 

 and "wrinkled," but the expression was of one kind only. 

 The experiment was made, and Mendel found that the 

 second filial (F 2 ) generation included plants, part of which 

 possessed only smooth seeds, while the others had only 

 wrinkled seeds (Fig. 41). "Transitional forms were not 

 observed in any experiment." This illustrates in a striking 

 way the difference between inheritance and expression, 

 for a character cannot appear in a plant (or animal) unless 

 the plant possesses the factor or factors for that character. 

 Now, except for the comparatively rare cases where 

 mutation occurs, the factors in the F 2 generation must have 

 been derived by inheritance from the germ-cells of the FI 

 generation; but the experiment shows that they did not 

 come to expression there. The same law is illustrated in 

 the crossing of a sweet variety of maize (having wrinkled 



