122 Peculiarities of Yellow Types [CH. 



2. The fact that in mice no yellow homozygote exists. 

 It is certainly a most surprising fact that mice cannot be 



bred true to yellow the more so since there is no difficulty 

 in producing pure yellow rabbits and pure red strains of 

 guinea-pigs. Among the ordinary phenomena of heredity 

 no quite parallel case is known, though it is possible that 

 pile (yellow and white) fowls are in the same condition^. 

 Cuenot's suggestion of the incompatibility between yellow- 

 bearing gametes female and male, is the only one yet 

 offered in elucidation of the phenomenon t. While admitting 

 that this account has great plausibility, I think that we 

 must not place complete reliance on a hypothesis to which 

 no adequate or thorough test can as yet be applied. The 

 idea of incompatibility between gametes has more than once 

 been introduced to deal with the genetic phenomena of 

 Sex (q.v.) and in that connection the case of yellow mice 

 should be remembered. 



3. The behaviour of the agouti factor G. 



The operation of this factor varies to some extent in 

 the different types. In all it can be carried by the yellow 

 variety, but in the rabbit alone are yellows which bear G 

 obviously different (having white bellies) from those which 

 do not (having blue bellies). It seems difficult to suppose 

 that this factor actually causes the appearance of white on 

 the belly and tail, yet the fact is well-established that such 

 yellow rabbits are really agoutis, or wild greys, wanting in 

 black. 



The relation of the yellow, due to the presence of the 

 yellow factor K, to the yellow in the bands, caused by the 

 agouti factor G, is at present quite problematical, but micro- 

 scopically these yellows are indistinguishable. 



Until the relations of chocolate to the other pigments in 

 the rabbit and guinea-pig have been more fully explored it 

 is scarcely possible to draw up a scheme of symbolic notation 

 representing the comparative compositions of the different 

 animals ; but adopting the gametic formulae given (p. 78) 

 for the various colours in mice we may tentatively suppose 

 that yellows exist of all compositions which would be pro- 

 duced by adding Y to each of those groups of symbols. 



* See also Basset Hounds (p. 128). 



t For more recent evidence see Appendix to Part I. 



