MORGAN, COAT COLORS IN MICE 97 



different from mice with uniform coat on the one hand and from albinos 

 on the other, and that they carry something that causes spotting, however 

 variable, to reappear in the second generation. It is well known that 

 crossing spotted animals to albinos does not increase the amount of white 

 in the spotted coat. On the contrary, if the albino is derived from a 

 colored race with a uniform coat, the first generation may be uniform and 

 the second variable to the same extent as though a uniform coat had been 

 used. It is clear that the albino condition is not related as such to the 

 spotted condition. The paradox seems to me to find its solution in the as- 

 sumption that the spotted coat is not segregated from the uniform coat as 

 a "unit character" in the germ cell, but that it represents a process which 

 occurs in the early cleavage of the egg when the A factor is separated 

 in certain cells from the C factor. In other words, some of the cells lose 

 the C factor and the regions derived from these cells are therefore white. 

 The well-known variability of the coat pattern that has been so difficult 

 to fix as such can be explained in this way, owing to the irregularities in 

 the process that leads to the loss of C, or to subsequent shifting of the 

 cells. Three corollaries follow from such an assumption : First, that there 

 is no "spotting factor" as such that segregates in the germ cells of the 

 hybrid animal. All the germ cells contain the factor for color producing 

 and color determining — the loss comes later in the ontogeny. It is this 

 tendency to separate late that is inherited, and I have no objection to 

 calling such a tendency a spotting factor, provided that we do not confuse 

 its method of action with what occurs in ordinary segregation. 



A second corollary is that the tendency to dissociate in the cleavage 

 stages may also be supposed to occur in the hybrid of the first generation, 

 but, since the other factor 3 that stands for uniform coat (i. e., no disso- 

 ciation) is also present, the separation may not normally be evident, al- 

 though in rats, as I have pointed out, even in the first generation the 

 occurrence of dissociation is manifest in certain parts of the body. 



A third corollary also follows. Since crossing spotted and uniform coats 

 leads to a production in the F 2 generation of a long series of intermediate 

 forms, we must infer that the tendency to dissociate has been modified by 

 crossing with the uniform coat. It has become modified in the sense that 

 it is often shifted to a later stage in the cleavage, hence the great varia- 

 bility observed in the second or F 2 generation. Cuenot and, later, Castle 

 have shown that the spotted condition may be gradually shifted in the 

 direction of selection. Cuenot showed by continued selection of 

 lighter colored mice (i. e., spotted mice with more white) that the char- 



s The doubleness of each factor makes such an assumption possible. 



