HEREDITY 



characters, and the results obtained had been too complex 

 for analysis. Mendel avoided this mistake. Only when he 

 was convinced that he knew exactly what would happen 

 under the simplest conditions, did he try to steer his way 

 through the complications introduced when two or three 

 character distinctions were under observation. 



The first important fact brought to light by Mendel's 

 experiments was that the two members of a given pair of 

 contrasting characters may express themselves differently 

 in the hybrids. In the pea, if a pure-breeding red-flowered 

 plant is crossed with a pure-breeding white-flowered plant, 

 the flowers of the hybrids are always red. Mendel called 

 the character that was expressed in the hybrid the dominant 

 trait. The other member he called recessive^ since it had 

 been suppressed only temporarily and appeared again in 

 later generations. Dominance has not proved to be a 

 universal phenomenon, however; in many crosses the 

 hybrids are intermediate between the parents in their 

 appearance. 



It was the behavior of the second hybrid generation 

 rather than the original cross that gave Mendel the clue 

 to the first real law of inheritance, now known as the Law 

 of Segregation. In this generation dominants and recessives 

 appeared in ratios which always approached the limiting 

 proportion three dominant to one recessive. These recessives 

 always bred true. The dominants, on the other hand, could 

 be divided into two types; one-third bred true, while two- 

 thirds behaved as did the first hybrid generation. 



It is plain that such results could be obtained if half 

 of the reproductive cells of each sex contained unit factors, 

 or genes, for the dominant character, and half contained 

 genes for the recessive character, provided the union of 

 these cells was a matter of chance. And such was Mendel's 

 interpretation. It involved several radically new concep- 

 tions. The body cells of an organism were assumed to 



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