No. 635] WHITE SPOTTING IN MICE 487 



namely, one near the lower limit of black-eyed white 

 spotting and possessing the dark modifiers. Such con- 

 fusion is not apt to occur in the progeny of single pairs, 

 for if the parents possess the modifiers dark black-eyed 

 whites will be produced, but also dark piebalds, leaving 

 an appreciable gap between the two types. 



The Nature of the Modifiers 

 The effect of the same modifying genes upon the ex- 

 pression of both black-eyed white and piebald spotting 

 furnishes certain information concerning the nature of 

 the modifying genes themselves. The main spotting 

 genes W and s have been found to be properties of dis- 

 tinct loci indifferent chromosomes (Dunn, 1920). There- 

 fore the gene or genes which modify both W and 5 must 

 determine the general conditions underlying the forma- 

 tion of pigment in the coat rather than specific condi- 

 tions associated with a particular spotting gene. From 

 the present evidence it appears to the writer that these 

 modifying genes alter the internal environment of en- 

 zyme and chromogen upon which the main spotting genes 

 W and s act to bring about their specific effects. The 

 darkening modifiers appear to increase the general 

 amount of color forming substance. In the presence of 

 such modifiers both W and s produce relatively less than 

 the normal amount of white spotting. 



The Source of the Modifiers 

 The genes modifying the amount of white spotting in 

 the mice used in these experiments appear to have come 

 from certain self mice with which black-eyed whites and 

 piebalds had been crossed. The black-eyed white stock 

 used was originally bred by Dr. J. A. Detlefsen and 

 reached this laboratory through a fancier. When first 

 bred inter se no grading records were kept, but I am cer- 

 tain that no black-eyed whites were produced which were 

 less than 70 per cent, white. This agrees fairly well with 

 the range of the black-eyed whites bred by Little. The 



