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 Fi generation consists 
entirely of tall peas, and in the F2 progeny there are approximately 3 
tall peas: 1 dwarf. Further tests in the F; generation showed that the 
dwarfs breed true to that character, they give only dwarf progeny. 
One-third of the talls, 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 Ff, 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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