Mendel's Law 



29 



the game result, so that a single example will suffice. 

 Furthermore, he discovered that it made no difference 

 whether the staminate parent was a dwarf and the pistil- 

 late tall, or vice versa, and so for all the characters used. 

 In other words, what are called reciprocal crosses gave 

 the same results. 



The progeny of a tall 

 parent and a dwarf parent 

 were all tall. This genera- 

 tion is known as the first 

 hybrid generation or the 

 Fi generation. When this 

 generation was inbred the 

 progeny was made up of 

 tall and dwarf individuals 

 in a ratio of 3:1. This 

 generation is known as the 

 second hybrid generation, 

 or the F2 generation. The 

 dwarf forms of the F2 

 generation subsequently 

 bred true, producing only 

 dwarfs. Of the tall forms 



one-third bred true and two-thirds split up in just such 

 a 3 : 1 ratio as did their immediate parents of the F, 

 generation. This may be expressed diagrammatically 

 (fig. 2). 



Mendel's explanation of this behavior involved 

 three theses which at that time were new to biology. 

 These theses must be kept distinct from one another. 



I. Independent unit characters. — This means 

 that an organism, although representing a morphological 



Fig. 2. ^Diagram illustrating 

 visible results of Mendel's experi- 

 ments. Cross between tall parent 

 (r) and dwarf parent (D) gives 

 hybrid progeny, which are all tall; 

 hybrid progeny inbred gives 3 : i 

 ratio in second hybrid generation; 

 inbreeding each of these four indi- 

 viduals separately gives for third 

 hybrid generation results indicated 

 in bottom line. 



