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THE AMERICAN NATURALIST [Vol. LIV 



piebalds used were obtained originally from fanciers. 

 Sixty-live such piebalds varied from approximately 50 

 per cent, white to per cent, white dorsally, with a mean 

 grade of about 22 per cent, white dorsally. When these 

 had been crossed with self mice 278 piebalds extracted in 

 the second generation averaged 8 per cent, white dor- 

 sally. Their range was approximately the same as that 

 of the original piebalds but the darker classes between 

 0-10 per cent, white had been greatly enlarged. It is 

 probably from this cross that the darkening modifiers 

 were introduced into the piebald stock, for the piebalds 

 used in all subsequent crosses were descended from these 

 extracted piebalds, most of which showed but little white 

 spotting. It might be supposed that the darkness of the 

 extracted piebalds could be explained on the basis of con- 

 tamination of the piebald gene during its residence in 

 self animals. That this is not the true explanation is in- 

 dicated by other results. The cross of black-eyed white 

 X piebald (Table II, Cross 1) yields black-eyed white 

 progeny with a mean grade of 82.8 per cent, and pie- 

 balds averaging 12.5 per cent. The cross Tvpe "A" 

 X piebald (Table II, Cross 3) yields black-eyed white 

 young with a mean grade of 84.8 per cent, white and 

 "spotted" young, with a mean grade of 11.6 per cent, 

 white. The means in the two crosses do not differ sen- 

 sibly. Yet Type "A" is heterozygous ['or self and the 

 young it produces should exhibit contamination if any 

 takes place. Certain of the selfs with which piebalds 

 were crossed originally, and certain selfs with which 

 black-eyed whites were crossed later to produce Type 

 "A"s appear to have contributed modifying genes which 

 could have had no expression in the self mice, since these 

 lacked the main spotting genes. Not all selfs are geno- 

 typically similar in respect to the modifiers, since from 

 crosses of black-eyed whites with certain selfs has arisen 

 a lighter strain of black-eyed white which is now being 

 bred, while from other selfs has come a darker strain of 

 black-eyed white which probably possesses the modifiers. 



