No. 632] SABLE VARIETIES OF MICE 



259 



mice with brown pigment in approximately the ratio 3 : 1. 

 Of the blacks about one third were self black; the others 

 were yellows, sooties and black sables with varying 

 amounts of black in the fur, much like the F 2 array in the 



TABLE III 

 Crosses of Black-and-Tan with Brown 













i 





ft 1 ' 



■j-j iH 



... 





25 

 26 



Blk.-tan X Br 



FiBTX FiBT 



FiBT X brown. .. . 

 BT X Br. and tan 6 

 FiBT X FiBT from 











black-and-tan X wild agouti cross. In this F 2 distribution 

 were seen, besides all grades of intermediates, segregates 

 of both sorts, viz., yellows of grade 1 and black-and-tans 

 of grade 6, although the latter did not retain their dark- 

 ness throughout life and when bred proved not to be pure 

 segregates. The F 2 mice with brown pigment were sim- 

 ilarly divided into yellows and brown sables with varying 

 amounts of brown in the fur and self brown in the ratio 

 2:1. Of the yellow-browns some were evidently counter- 

 parts of black-and-tan with the black pigment replaced by 

 brown. To these the name brown-and-tan was given; 

 the other yellow-browns paralleled the yellow-black series 

 although the lower and intermediate grades were less well 

 represented, due perhaps to the difficulty of distinguish- 

 ing between yellows with considerable brown pigment and 

 yellows with smaller amounts of black pigment. Breed- 

 ing tests were used in doubtful cases. 



The Fj light black-and-tans, when backcrossed to 

 browns (Table III, cross 24), gave equal numbers of all 

 four sorts, viz., yellow-blacks, self blacks, yellow-browns 

 and self browns. The mode of these yellow-blacks was in 

 the middle class (grade 3) in contrast with the mode at 

 5.5 in the straight F 2 . 



These crosses of black-and-tan with brown show the in- 

 dependence of the darkeners from black pigment for in 

 these experiments the modifiers have been detached from 



