HEREDITY 321 



spring through several generations. These variations he recognized 

 as alternative; that is to say, in peas Tall and Dwarf are alternative 

 height characters; Yellow and Green, Smooth and Wrinkled are 

 alternative seed characters, and so on. We recognize alternative char- 

 acters in human inheritance; for instance, blue and brown eye color 

 are alternative. 



Monohybrid Cross. We will discuss here only illustrative 

 cases which will serve to demonstrate the principles that Mendel's 

 work uncovered. First, attention will be confined to a single pair 

 of alternative characters. Tall and Dwarf. Mendel found that when 

 two plants of these varieties were crossed, the first generation, desig- 

 nated as Fi, were all Tall. The progeny of the crossing of two varie- 

 ties are called hybrids. Since in this case the varieties differed in 

 only one character, height, the offspring are termed monohybrids. 

 The first filial generation being all like the Tall parent, we note 

 at once that Gallon's law, that offspring inherit equally from both 

 parents, does not appear to be valid. When two of these mono- 

 hybrids of the Fi generation were crossed, the second filial genera- 

 tion, F2, consisted of a large number of individuals of different 

 sorts. Three-fourths of the number were Tall like one grandparent, 

 and the remaining fourth were Dwarf. When two of these Dwarfs 

 were crossed, all their progency were Dwarf in all succeeding 

 generations, Tallness having dropped completely out of that line 

 of descent. Nor were these Dwarfs in any way different from the 

 original Dwarf parent used in the first cross. In other words, the 

 one-fourth that were Dwarfs in the F2 generation (that would be 

 the grandchildren in a human family) were pure for this char- 

 acter and Dwarfness had been introduced into a generation (Fi) 

 and extracted again without change (Fig. 204). 



Breeding experiments with the other three-fourths of the F2 

 generation showed that they consisted of two types, genetically, 

 although all appeared alike outwardly. One out of every three, 

 when self bred, gave progeny that were all Tall and continued to 



