CHAPTER LXXX 

 HEREDITY 



467. The problem of heredity. How is it that the characters 

 of the parents are transmitted so regularly to the offspring ? 

 How is it that, in spite of the close resemblances between par- 

 ents and offspring, these are never exactly alike in every point ? 

 These questions have to do with the problem of heredity. 



468. Analysis of the problem. Certain facts, or laws, of 

 heredity were first discovered in part by an Austrian monk 

 named Gregor Mendel (1822-1884). Mendel had long puzzled 

 about the great variations among his garden peas. There were 

 tall plants and short ones, plants with white flowers and plants 

 with colored flowers. In some plants the seeds were yellow, in 

 others they were green ; some seeds were smooth, others were 

 wrinkled. All in all, he studied seven different pairs of con- 

 trasting characters in regard to which pea plants differ. He 

 noticed further that a given plant might have any combination 

 of single members of these pairs. A hairy plant might be tall 

 or it might be short, it might have yellow seeds or it might 

 have green seeds, it might have full pods or shrunken pods, 

 and so on. 



469. Mendel's experiments. Fixing his attention on a single 

 character at a time, instead of trying to think of the variety as 

 a whole, Mendel crossed garden pea plants that were different. 

 Thus, he crossed green-seeded plants with yellow-seeded ones, 

 tall ones with short ones, hairy ones with smooth ones, and so 

 on for all the pairs of differences. 



When green-seeded plants were crossed with yellow-seeded 

 plants, the seeds in the next generation were all yellow 

 (see Frontispiece). 



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