490 



THE AMERICAN NATURALIST [Vol. .LIV 



while 33, or about 6 per cent., showed one or more white 

 hairs. The spotting in the F 1 mice was confined to the 

 ventral surface, 5 usually in the center of the belly. The 

 minimum size of this spot was a few white hairs, its 

 maximum extent was 12 per cent, of the ventral surface, 

 and its average extent was 3 per cent, of the ventral sur- 

 face. All young produced by this cross must be regarded 

 as selfs since all produced young, when interbred, in the 

 ratio of three selfs to one piebald. What, then, is the 

 cause of the appearance of certain animals in F x which 

 show some characteristics of the recessive parent? 



There may be two answers to this question: (1) the 

 apparent imperfections of dominance in F x may be due 

 to fluctuations in the somatic expression of the piebald 

 gene when present singly in the zygote; (2) they may be 

 due to a definite gene or genes for a small amount of 

 ventral spotting in mice heterozygous for piebald. 



As evidence for the first view we have relevant data 

 in the experiments just reported. An analysis reveals 

 that the production of young with small amounts of 

 white spotting is not correlated with the somatic appear- 

 ance of the piebald parents since in amount and distribu- 

 tion of spotting these parents as a class are not distin- 

 guishable from the parents which produced only true 

 self young. On the other hand, practically all the spotted 

 F^ which were produced by individual piebald parents 

 had for their other parent a particular self animal. Ap- 

 parently the selfs as well as the piebalds varied in the 

 power of producing slightly spotted young. Causes in- 

 fluencing the production of this small amount of spotting 

 in F x may have been contributed by the self parents, al- 

 though the posibility that part of the causes came from 

 the piebald parents is not excluded by the evidence. If 

 this be accepted as evidence that selfs share in the pro- 

 duction of slightly spotted mice, then certain selfs must 

 differ genetically from other selfs, and the point of dif- 

 ference may be a separate recessive factor for ventral 



o With the exception of the few dorsally spotted mice referred to on p. — . 



