CHAPTER II 



MENDEL'S FIEST LAW— SEaEEGATION 

 OF THE GENES 



Mendel succeeded in discovfering the principle of 

 segregation because he simplified the conditions of his 

 experiments so that he had to deal with one process at 

 a time. Others before him had failed because they worked 

 with too complex a situation. In each case Mendel picked 

 out for study a pair of contrasted characters of a kind 

 that were sharply distinguishable from each other when- 

 ever they appeared. He chose plants that normally self- 

 fertilize and are little liable to accidental cross-fertiliza- 

 tion, which made it possible easily to obtain in the second 

 generation numbers large enough to give significant 

 results. To Mendel's foresight in arranging the condi- 

 tions of his work, as much as to his astuteness in interpret- 

 ing the data, is due his remarkable success. 



Mendel used varieties of the common edible garden 

 pea (Pisum sativum). Many of these varieties (races) 

 differ from each other in a particular character. Some 

 races are tall, others short; some have green peas (seeds 

 in the pods), others have yellow peas ; some of these seeds 

 have a smooth surface, others are wrinkled; some of the 

 pods are hard, others are soft. One of the crosses made 

 by Mendel will serve as an illustration of his work (Fig. 1) . 



Pollen from a race of tall peas was put artificially on 

 the stigma of a plant of a short race, whose own stamens, 

 and therewith the pollen, had been previously removed. 

 The hybrid plants that came from the seed were tall. 

 These hybrids were allowed to self-fertilize and their 

 seeds collected. Some of the seeds produced tall plants, 



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