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 o] 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 Fa 
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. 
