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THE AMERICAN NATURALIST [Vol. XL VIII 



chocolates. 3 The gray white-bellied offspring were se- 

 lected and these were bred again to chocolate. The cross, 

 in regard to sex, was made both ways. If there is an 

 independent factor for white-belly that can separate from 

 the factor for gray gray-belly, then some gray gray- 

 bellied mice should appear. None were obtained, as the 

 following table shows. We must conclude either that 

 there is one factor that gives the gray white-bellied coat, 

 or else that the postulated factor for white-belly is so 

 closely linked to the gray factor that it has not sepa- 

 rated once in 100 times. Therefore unless such a sepa- 

 ration occurs it is simpler to assume one factor for gray 

 white-belly that is allelomorphic to black and to gray 

 gray-belly, etc. 



TABLE I 



Taking both crosses together, there are 44 grays to 54 

 blacks and chocolates, which approximate at least to 

 expectation. To these numbers I may add the follow- 

 ing data taken from similar experiments made for other 

 purposes in which one parent was, as before, gray white- 

 belly. 



17 $ ^20 



Presumably, therefore, the results may be treated as 

 though a single gene for gray white-belly exists. It will 

 be observed that the experiment has been made in two 

 ways, for at the time I was aware of the possibility that 

 crossing over, if it occured, might be limited to one sex. 



» At the time when the experiment was made all the gray white-bellied 



