MENDELIAN HEREDITY 277 



of environment upon thoin, as must happen when wild plants 

 are used for experiment, and they were sufficiently variable to 

 afford abundant confirmation of his conclusions. He could hardly 

 have made a happier choice for pioneer genetic research. The 

 characters with which he dealt were : 



1. Shape of seeds. Some peas are round or nearly so and smooth 

 when ripe. Others have their seeds shrunken into deep wrinkles. 



2. Color of cotyledons. These seed leaves make up the greater 

 part of the pea seed, and may be yellow or green. 



3. Color of seed coat. Some seeds have a white coat and are 

 produced by plants with white flowers, while others have gray to 

 brown coats and are developed from purplish flowers. 



4. The form of ripe pods. These may be either inflated or 

 wrinkled and deeply constricted between the seeds. 



5. Color of unripe pods. Green or yellow. 



6. Position of flowers. Axillary or terminal. 



7. Length of stem. Plants vary considerably according to envi- 

 ronment, but under similar conditions peas fall into two classes. 

 There are tall or climbing peas which may be several feet in 

 height and dwarf peas which attain a maximum length of about 

 eighteen inches. 



Methods. By carefully controlled cross-fertilization Mendel 

 produced hybrids of these varieties. For example peas bearing 

 smooth seeds were crossed with those bearing wrinkled seeds. 

 The result was the production of smooth seeds only, and in order 

 to determine the fate of the other character these were planted, 

 the plants self-fertilized, and a second generation of seeds produced. 

 In other experiments plants bearing y(>llow seeds were crossed 

 with those bearing green seeds, tall plants with short plants, plants 

 with yellow cotyledons and those with green, and a number of 

 others. The behaviour of these characters in the hybrid was noted, 

 and the nature of their reappearance in successive generations. 



Monohybrids. In the hybrids which Mendel produced only 

 one of the two characters involved was evident. The hybrid peas 

 of the smooth-wrinkled cross, for example, were all smooth. When 

 the plants developed from these smooth seeds were self-fertilized, 

 thus com])ining only the types of gametes which a hybrid could 

 produce, the missing character appeared again. Approximately 

 one quarter of the resulting seeds were wrinkled. On raising yet 

 another generation the three quarters which were smooth were 



