PRINCIPLES OF GENETICS 3 



evident that when both are brought together the tall 

 dominates the short, or, conversely, short is recessive to 

 tall. 



Mendel pointed out that the 3 to 1 ratio that appears 

 in the second generation can be explained by means 

 of a very simple hypothesis. If the element for tall 

 and the one for short (that are both present in the hy- 

 brid) separate in the hybrid when the eggs and pollen 



^^^' 



short short 



Fig. 2. 



A "back-cross" of Fj hybrid, tall (short) peas to the recessive 



type (short), giving equal numbers of tall and short offspring. 



grains come to maturity, half the eggs will contain the 

 tall and half the short element (Fig. 1). Similarly for the 

 pollen grains. Chance fertilization of any egg by any 

 pollen grain will give on the average three tails to one 

 short ; for, when tall meets tall a tall plant is produced ; 

 when tall meets short a tall plant results; when short 

 meets tall, a tall plant is produced ; and when short meets 

 short, a short plant arises. 



Mendel put this hypothesis to a simple test. The hybrid 

 was back-crossed to the recessive type. If the germ-cells 

 of the hybrid are of two kinds, tall and short, there should 



