EXPERIMENTAL STUDY OF HEREDITY 555 



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 oj 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 all 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. 407). " Transitional forms were not 

 observed in any experiment." This illustrates in a strik- 

 ing way the difference between inheritance and expression. 



475. Ratio of Segregation. But now we come to that 

 feature of Mendel's experiments which, perhaps more than 

 anything else, made them superior to all others that had 

 preceded. He carefully counted the number of plants 

 bearing each kind of seed, and found that the number 

 of smooth-seeded plants was to those with wrinkled 

 seeds as 3 :i. 



476. Theory of Purity of Gametes. When the wrinkled 

 seeds (one-fourth of the total crop) were sown they all 

 bred true to wrinkledness their descendants of the F 3 

 generation bearing only wrinkled seeds. The expression 

 was alike in every case. The gametes that united to 

 produce these plants were therefore considered pure for 

 "wrinkledness;" that is, it was inferred that they did 

 not carry any inheritance tending to produce smoothness 

 of seed. 



