THE MENDELIAN PRINCIPLES OF HEREDITY 75 



preciated the significance of his work, and realized 

 that he had found the key to the discoveries that they 

 too had made. From this time on the recognition of 

 Mendel's discovery as of fundamental importance was 

 assured. Bateson's translation of his paper made 

 Mendel's work accessible to English biologists, and 

 Bateson's own studies showed that Mendel's principles 

 apply to animals as well as to plants. 



THE HEREDITY OF ONE PAIR OF CHARACTERS 



Mendel's discovery is sometimes spoken of as Men- 

 del's Principles of Heredity and sometimes as Mendel's 

 Law. The former phrase gives a better idea perhaps 

 of what Mendel really accomplished, for it is not a little 

 difficult to. put his conclusions in the form of a law. 

 Stated concisely his main discovery is this : — in the 

 germ-cells of hybrids there is a free separation of the 

 elements derived from the two parents without regard to 

 which parent supplied them. 



An example will make this more obvious. Mendel 

 crossed an edible pea belonging to a race with yellow 

 seeds to a pea belonging to a race with green seeds 

 (Fig. 42). The offspring or first filial generation (Fi) 

 had seeds all of which were yellow. When the plants 

 that bore these seeds were self-fertilized, there were 

 obtained in the next generation, F2, both yellow and 

 green peas in the proportion of 3 yellows to 1 green 

 (Fig. 42). This is the well-known Mendelian ratio 

 of 3:1. 



The clue to the meaning of this ratio was found when 

 the plants of the second generation (Fo) were selfbred. 

 The green peas bred true ; but the yellows were of two 



