cii. IV] The Ratio 9:3:4 75 



out experiment no prediction can be made with confidence 

 as to the behaviour of such types in their crosses. 



Albinism being recessive in all ordinary cases, F^ from the 

 cross colour x albino contains i albino to 3 coloured members. 

 As regards the characters of the dominant or coloured mem- 

 bers various complications have to be considered. 



In the simplest cases the coloured F^ individuals are all 

 of the same colour. For example on crossing a grey rabbit 

 with an albino, F^ is grey and F^ may be 3 greys : i albino. 

 But frequently it is found that in addition to the greys and 

 albinos blacks appear in F^, Repeated experiments, for 

 example those of Hurst, have shown that in such families 

 the F^ ratio is 



9 greys : 3 blacks : 4 albinos 



^ , ' 



12 : 4 



3 '- I 



The relation of this ratio to the ordinary 9:3:3:1 

 was first pointed out by Cuenot (86) '^. As represented by 

 him two pairs of allelomorphs are concerned, namely : 



Dominant, Recessive. 



1. Colour (C), Albinism {A). 



2. Grey determiner (6^). Black determiner (^). 



The presence of one or other of the determiners G ox B 

 is only perceptible when it exists in combination with the 

 colour- factor. If 6^ is present together with C, the colour 

 is grey ; if ^ is present with C but without Gy the colour is 

 black. If a coloured individual contain both G and B, 

 being thus heterozygous in the second pair of factors, the 

 colour is grey, for the effects of grey dominate. But since 

 in the absence of colour (C) neither determiner produces a 

 perceptible effect, albinos may exist of the forms AAGG, 

 AAGB, or A ABB, and without breeding tests it will not 

 be possible to distinguish between these several forms. On 

 crossing with a black of course each albino can be known 

 by its effects. For GG will then %\v^ greys only, GB will 

 give equal numbers of greys and blacks, while BB albinos 

 will give only blacks. 



* Shortly before the publication of Cuenot's paper Mr R. H. Lock 

 wrote to me from Ceylon with the same suggestion. 



