CHAPTER XXIX 

 THE LAW OF INDEPENDENT ASSOCIATION 



The Independence of Characters. — For the sake 

 of simplicity in the discussion of Segregation our attention 

 has been confined to a single pair of characters and their 

 corresponding genes. Of course the two parents often 

 differ in respect to several character pairs. What is true, 

 however, in respect to one pair is also true of any other 

 pair. In order to illustrate the independence of characters 

 in expression and in heredity at least two character-pairs 

 are necessary. If a pure tall white-flowered pea is crossed 

 by a pure short purple-flowered one the hybrid offspring 

 are all alike and all tall and all purple-flowered. In this 

 it is to be noted first of all that neither parent has domi- 

 nated over the other as a whole. The hybrid has inherited 

 its height from one parent and its flower-color from the 

 other. If a tall purple is crossed by a short white the 

 hybrid is again tall purple. In this case it resembles one 

 parent exclusively. From the two experiments it becomes 

 clear that the expression of particular genes depends on 

 their own powers and not on which parent they have 

 been derived from. 



The Combinations of Independent Characters in 

 F2. — If the hybrid tall purple peas discussed above are 

 self-pollinated to produce a second hybrid generation it 

 will be found to consist of approximately 9 tall purple; 

 3 tall white; 3 short purple; 1 short white. It is to be 

 noted that two of these combinations are the same as the 

 grandparents and two are new combinations. The result 

 is the same, whether the tall purple hybrids have been 

 produced by crossing tall whites by short purples or tall 

 purple by short white. Thus not only do these pairs of 

 genes for tall-short and purple-white express themselves 



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