36 



THE INHERITANCE OF COLOR IN MICE, 



further complicated by the occurrence of so-called "reversed sables," in which 

 the dark hairs had yellow tips and bases, with a dark subapical band. 



The writer must confess that he is unable to draw a clear line between 

 "sables" and "sooty yellows" in all cases. The term sooty yellow is there- 

 fore intended in this paper to cover both varieties. 



It is possible to explain the appearance of sooty yellows on the basis of 

 fluctuation in activity of the factor R, restriction. Thus, when R is present in 

 full strength the animal is clear yellow; if R is slightly weaker, some dark 

 pigment shows itself. 



The amount of dark pigment visible becomes progressively greater as the 

 factor R becomes weaker or takes on a rhythmical activity. Thus, if R is 

 hampered in activity during the growth of the first part of the hair, we have a 

 dark tip formed. But if R comes into full activity again, a yellow band results, 

 followed by a second weakening of R, resulting in a dark base to the hair. 

 This would account for the superficial similarity to the agouti pattern. "Re- 

 versed sables" may possibly result from a reversal of the order in which the 

 strong and weak periods of R occur. So, too, those very dark sooty yellows 

 that lack the ticked-hair pattern may be considered due to continued weakness 

 of the factor R during development of the hair of the first coat. When the 

 second coat is formed, however, the factor R makes its appearance felt more 

 strongly and an ordinary type of sable results, with less dark pigment than was 

 present on the first coat. 



In all such cases the idea is present that there is quantitative variation in 

 the activity of the restriction factor R, showing itself in the varying amount of 

 black or dark pigment present. If such weakness is due to any interchange of 

 substance between gametes or any quantitative inexactitude in the process of 

 segregation, we should expect that repeated crosses of sooty yellows with 

 blacks would tend to produce much weakened yellows {i. e., very dark sooty) 

 with much black pigment, as well as modified blacks with some yellow pigment 

 present. While this experiment has not yet been carried to large numbers it 

 may be of interest to record the pedigree of one family: 



*Among these sooties was cflBlS (fig. 20) graded +3, darker by four "one-quarter" 

 grades than any sooty previously obtained. The best previous was +2. Among the blacks 

 in the same litter was cflSlG, the first visibly modified black known to be homozygous in 

 hundreds of black animals that I have seen. His coat is distinctly tawny. 



"Sooty yellows" and "sables" have not been reported in the pink-eyed 

 series. This accords with the results expected by theory, for the modification 

 of the factor P, which produces pink-eyed forms, reduces the brown and black 



