CHAPTER X 

 FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 



By quantitative inheritance is meant inheritance which has to do 

 with the size of organisms or parts of organisms, with the number of 

 certain parts, or at times with shapes and forms as related to size. The 

 category of quantitative inheritance is one of convenience only and is not 

 separated sharply from other forms of inheritance. It is, however, in 

 general notable for its complexity and difficulty of precise analysis, and 

 these features of it have resulted in diverse interpretations, some in 

 harmony with the Mendelian principles which have been discussed 

 up to this time, and others which call upon auxiliary hypotheses for 

 aid. In this discussion, we shall begin with simple cases of size in- 

 heritance and proceed from them to others more complex. Throughout 

 the attempt will be made to develop a consistent explanation for the 

 phenomena, and one which is in harmony with the general explanation 

 advanced in Mendelian heredity. 



Mendel's original experiments dealt with one case of size inheritance. 

 When a tall pea is crossed with a dwarf pea, the F\ generation consists 

 entirely of tall peas, and in the F z progeny there are approximately 3 

 tall peas : 1 dwarf. Further tests in the F 3 generation showed that the 

 dwarfs breed true to that character, they give only dwarf progeny. 

 One-third of the tails, also, breed true to the tall character, but the 

 other two-thirds give progenies which display segregation into tall 

 and dwarf in the ratio originally obtained in the Ft generation, namely 

 3 tall : 1 dwarf. The results throughout are, therefore, in harmony 

 with the assumption that there is a single factor difference between tall 

 and dwarf peas and that the factor for tall is completely dominant over 

 the dwarf factor. This case we have treated in detail in Chapter V. 



Results similar to this are not uncommon in studies in size inheritance 

 involving differences between tall and dwarf races. They have been 

 reported for tall and dwarf tomatoes, sweet peas, maize, beans, snap- 

 dragons, etc. In the case of beans, Emerson has pointed out that the 

 differentiating factors are factors for indeterminate as opposed to a deter- 

 minate habit of growth. In the dwarf or bush type of bean, illustrated 

 by Fig. 82 (right), the axis of the plant is terminated after the formation 

 of from four to eight nodes by an inflorescence. Pole or tall beans, as is 

 shown on the left in Fig. 82, do not have such a terminal inflorescence 



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