78 Presence and Absence [CH. 



In this diagram the 9 squares containing C, G, B, are 

 the 9 greys, the 3 squares containing C and B only are the 



3 blacks and the 4 squares containing no C at all are the 



4 albinos. 



Proceeding to the case of mice we write the composition 

 as follows : 



Grey C, G, B, Ch. 



Black C, g, B, Ch. 



Chocolate C, g, d, Ch. 



We thus regard the black mouse as one from which the 

 grey determiner. G, has been removed. In the chocolate 

 mouse the process of removal has been carried further and 

 the black determiner, B, is also gone. 



A proof that this system of representation is so far 

 correct is obtained by crossing the grey mouse with the 

 chocolate. Such a cross, if G is not allelomorphic to B, must 

 give blacks in /%. This experiment has been lately carried 

 out by Miss F. M. Durham, to whose work our knowledge 

 of the genetics of mice is largely due. The result is that, 

 as expected, /% does contain blacks, and though the num- 

 bers as yet obtained are small, there can be little doubt 

 that the /% ratio is 



12 grey : 3 black : i chocolate. 



Some interesting questions arise in regard to the greys. 

 Obviously we should expect 9 greys containing G and B 

 + 3 greys without B. Now fanciers are well aware of a dis- 

 tinction between two kinds of greys or "agoutis" as they are 

 called. These are known as "golden agoutis" and "cinna- 

 mon agoutis," the former containing black pigment, the 

 latter being without it. In the F,. from grey x chocolate 

 both these kinds of agoutis appear, and evidently the 

 cinnamon agoutis are the expected greys wanting in the 

 determiner B. 



Thus far all is clear. Certain difficulties however 

 remain unexplained. These will be described later. At 

 this stage in the discussion it is convenient to notice that 

 in view of the facts now stated the use of the term domi- 

 nance must be more carefully restricted than has hitherto 

 been necessary. When we speak of the colour as being 

 dominant over the absence of colour we mean that if the 

 colour is present it will appear, and that if the factor for 



