CHAPTER IV 



MENDEL ^S SECOND LAW— THE INDEPENDENT 

 ASSORTMENT OF THE aENES 



Mendel proved that when races differ from each 

 other in two pairs of characters, each pair considered 

 by itself alone gives the 3 : 1 ratio, and the inheritance 

 of one pair is independent of that of the other. If a tall 

 race of peas with colored flowers is crossed to a short 

 race with white flowers the offspring show the two domi- 

 nant characters, i.e., they are tall and have colored 

 flowers. If these are inbred they produce tall and short 

 offspring {F2) in a ratio of 3: 1, and these same individ- 

 nals, if reclassified for pigment, are colored or white in 

 the ratio of 3 : 1. For example, the ideal for 12 tall peas 

 would be 9 colored and 3 white; and for 4 short peas 

 there would be 3 colored and 1 white. Expressed in a 

 diagram we have : 



12 tall 4 short 



9 colored : 3 white 3 colored : 1 white 



The preceding way of stating the results deals 

 directly with the facts. The explanation, of these results, 

 based on the segregation of the members of the two 

 independent pairs of factors, is as follows: If we call 

 the gene for tallness by the same name as the character 

 itself, viz., tall, and the gene for shortness by the name 

 of this character, viz., short, and similarly for the other 

 pair of characters, viz., color versus white, then when 

 crossed the hybrid has two pairs of allelomorphs, 



tall color 



short white 



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