192 GENETICS IN RELATION TO AGRICULTURE 



and intergrading with the parental forms, escaped notice as mutations 

 but were selected for continuing the experiment? Such an assumption 

 would render intelligible the efficacy of return selection which would be 

 difficult of interpretation on even a multiple factor theory of heredity. 



That such a system may exist in qualitative characters has been 

 shown by Bridges for the relation between eosin eye color and its modifiers 

 in Drosophila. One modifier called dark intensifies the eosin character. 

 The other six modifiers are all diluters. Thus cream a changes eosin to 

 pale yellow or cream color, cream b has a similar effect, but not so marked. 

 Whiting changes the eosin color to white, so that eosin-whiting flies are 

 indistinguishable from white-eyed flies in color. In these cases there is 

 no question as to the operation of a multiple system of factors, for the 

 specific factors have arisen singly by mutation and their linkage relations 

 establish completely their identities. Nevertheless taken together they 

 would give in a qualitative character a remarkably close imitation of 

 the behavior of Castle's hooded rats. 



If, however, we assume with Castle that factors like characters are 

 variable and that allelomorphic contamination occurs, then we may offer 

 an explanation based on a consideration of a single allelomorphic system. 

 For such an explanation the hooded pattern may in general be represented 

 by h, and its dominant allelomorph, the fully colored condition, by H, 

 Self-color is dominant to hooded, but the hooded condition varies greatly 

 in the amount of pigmentation present in the coat. These variations 

 appear to be correlated with definite factor variations, consequently we 

 may designate the factors determining the various degrees of pigmen- 

 tation in the hooded pattern by hi, hz, hs, h*, . . . h n . This series runs 

 from individuals which show practically no color to those which display 

 almost a self-colored coat. If we assume that the character expression 

 of an animal of the genetic constitution hihio be intermediate between 

 that of an animal of the genetic contitution hihi, a very light type, and 

 one of the constitution hiohio, a very dark type, then we may point out 

 what would occur if selection were carried out in the progeny of such 

 an individual. In the first place the genetic constitution hihio of such an 

 animal represents merely the values of the gametes that united to form 

 the zygote. They are assumed to interact immediately, so that perhaps, 

 in addition to the factors hi and AI O , such a zygote will produce gametes 

 bearing for the most part the factors hs and h$, representing a sort of 

 equilibrium for the interaction of the factors hi and hi . There would, 

 therefore, be in the progeny of such an individual some individuals of 

 the genetic constitution h^ which would be lighter than the parents, and 

 some of the genetic constitution h 6 hi which would be considerably darker 

 than the parents. If other products of this reaction, such as h 3 , ft 4 , hi, h s , 

 etc., were also produced, and like the original reacting factors hi and hi 



