FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 189 



2. Typical Blending Inheritance. Factors o r allelomorphic characters 

 blend in the heterozygote to form factors of intermediate character. 

 The factors of the heterozygote are as uniform as those of either parent 

 individual. 



3. Partial Blending Inheritance. Factors of allelomorphic characters 

 segregate in gametogenesis in the heterozygote, but with modification 

 due to varying degrees of blending between the two allelomorphic factors. 



The first category here set off is that of ordinary Mendelian inherit- 

 ance, and includes all cases in which there is admittedly no contamination 

 by association of the members of an allelomorphic pair in the same 

 individual. The second category calls for an assumption of factorial 

 alteration to such an extent that all the factors of the parents as such 

 disappear and only a factor determining an intermediate condition re- 

 mains. The case may be illustrated abstractly in this fashion. Let the 

 difference between two races genetically be that between A and a. 

 The races are crossed. Now according to ordinary Mendelian assump- 

 tions the factors A and a will segregate in the germ cells of the offspring 

 and without factorial contamination. According to the assumption of 

 typical blending inheritance, however, the two factors A and a when they 

 meet in the hybrid immediately interact and this interaction gives an in- 

 dividual having the genetic constitution A' A' rather than A a, the symbol 

 A' representing a factor intermediate in its character expression between 

 A and a. As a consequence of this change in the factors involved such 

 an individual, although of hybrid origin, is genetically not a hybrid and 

 would consequently breed true. Admittedly the cases which fall into 

 this category are not common, and it is a matter of debate whether any 

 have thus far been found. As was pointed out in connection with the 

 discussion of the conditions for a Mendelian interpretation of quantita- 

 tive inheritance, the same relations find a consistent Mendelian explana- 

 tion in those cases in which the number of factor differences is very large. 



The third category assumes that a variety of conditions may arise as 

 the result of the production of an A a individual. The blending may be de- 

 finite so that A and a become A' and a', or it may be indefinite and give 

 rise to a series of factors Ai, A%, As, . A n and ai, a z , a 3 , . .' . a n , 

 all -of which may be represented in the gametes of the heterozygous in- 

 dividual. When definite factor contamination is assumed it may be 

 regarded simply as an expression of the condition that the reacting 

 system Aa reaches an equilibrium when it becomes A 'a', a condition for 

 which there are many analogies in chemical reactions. In case some of 

 the original factors still remain, such a heterozygous individual would 

 produce gametes A, A', a', and a, which by conjugation would produce a 

 variety of forms in its offspring. In such a case an individual AA', or Aa' 

 might give rise to the establishment of a new equilibrium, but equally 



