38 Outline of Genetics 



parent 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 generation is known as the first hybrid 



or the Fi generation. When this generation was inbred, 



the progeny was made up of 



tall and dwarf indi\dduals 



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 

 visible results of Mendel's experi- ^^^^^^^ q^ ^^^ ^^^j ^^ 

 ments. Cross between tall parent 



(T) and dwarf parent (D) gives one-third bred true and 



hybrid progeny which are all tall; tWO- thirds split up in juSt 



hybrid progeny inbred gives 3:1 guch a 3:1 ratio as did 

 ratio in second hybrid generation; ... ,. . 



inbreeding each of these four indi- their unmediate parents of 



viduals separately gives for third the Fj generation. This is 



hybrid generation results indicated expressed diagrammati- 

 in bottom line. ,, . ^ 



cally m fig. 2. 



Mendel's theoretical explanation of this behavior 

 involved three distinct theses. 



I. Independent unit characters. — This means 

 that an organism, although representing a morphologi- 

 cal and physiological unity, from the standpoint of hered- 

 ity is a complex of a large number of independent herit- 

 able units. Thus if one pea plant is tall and another one 

 is dwarf, the behavior of the hybrid produced from them 

 with reference to this character will be the same, no 



