No. 035] 



WHITE SPOTTING IN MICE 



479 



servations by the writer and all data in the tables and 

 figures of this report have been tabulated according to 

 these ranges. Such a table is approximate and may be 

 only temporary, for the breeding of larger numbers of 



Summary of Range Data 

 table I 



spotted mice may increase the range now attributed to 

 each type. Whether the ranges of variability represent 

 the norm for each type is not at present known, nor can 

 it be known until each type by inbreeding or other suit- 

 able methods has been separated from the subsidiary 

 factors which, as it will appear, alter the expression of 

 the main spotting factors. 



Modifying Factors 



The general result of the foregoing discussion is sim- 

 ply an exposition of the great amount of variability ex- 

 istent within each type of spotted mice, all of which are 

 identical as regards the main genes now known to deter- 

 mine white spotting. The question naturally arises, if 

 a mouse with a coat entirely white except for pigment in 

 the eyes and a mouse in which the dorsal surface is 

 equally divided between pigmented and white spaces are 

 genetically identical as regards the main genes W and s, 

 how then do thev differ? Is each merely a somatic va- 

 riation (fluctuation) of a genetic complex determining a 

 combination of colored and white spaces halfway between 

 these extremes? Or do other genetic factors in com- 

 bination with the main spotting genes determine greater 

 or less amounts of pigment in the coat? 



If we adopt the fluctuation hypothesis then we must 



