The progress of Genetics since the rediscovery of Mendel's papers. 385 



tiou of two imaginary pairs of allemorphs, C, c and K, r, the small 

 letter denoting the absence of that factor whose presence is denoted 

 by the large letter. If R cannot be perceived unless C is also present, 

 there must be 9 members showing CR. 3 showing C, and 4 all in- 

 distinguisliable seeing tliat they only differ in the presence or 

 absence of R. 



Fig. 13. Scheme of distribution of two pairs of factors in F?, showing how the F.; 

 ratio 9:3:3:1 is produced. From the same distribution the Fo ratio 9:3:4 results 

 in cases where one factor produces no visible appearance unless the other is present : 

 while, if neither factor can be perceived unless both are present, the F^ generation 

 is 9:7. The big letter in each case denotes the presence, the small letter the 

 absence, of the given factor. 



So long as we are only concerned with two colour-determiners 

 giving two coloured types, in addition to the albino, this mode of 

 representation is adequate. But it may happen, as in the case of 

 mice, that more colours have to be represented. Here for example, 

 even excluding yellow, which as Cuénot (42c) has shown presents 

 special complexities, we have to deal with (I) the wild grey (2) black 

 (3) chocolate. The facts in this case can be represented by a modi- 

 fication of Cuénot's system, which there is considerable reason to 

 suppose corresponds more precisely with the facts. Here, as before, 

 we take C and c representing presence and absence of colour, but 

 instead of regarding B the black determiner, as allelomorphic to G, 

 the grey determiner, each is taken to be allelomorphic to its own 

 absence, thus: 



G, grey determiner 

 B, black „ 



Ch, chocolate 



g, absence of ditto 



"> 55 55 55 



ch ^) 



^"j )5 55 55 / 



As a matter of experiment no case is yet known in which Ch is absent. 



25* 



