L 



CHAPTER VII 



HEREDITY OF COLOUR— continued. 



The Genetics of Yellow Pigments in certain Animals. 

 Yellow Mice not breeding true — The Case of Basset 

 Hounds and the ''Law of Ancestral Heredity'' 

 Relation of this Principle to Mendelian Rules. 



The genetic relations of yellow and black pigments 

 present so many difficulties that a separate chapter is 

 needed for their consideration. There is one comparatively 

 simple case, that of the thorough-bred or race horse, in 

 which the presence of black behaves as a simple dominant 

 to the colour known as chestnut. In practically all the 

 other forms that have been investigated there is some 

 complication. One of these is the celebrated example of 

 the Basset Hounds, which played so prominent a part in 

 connection with Galton's enunciation of the "Law of 

 Ancestral Heredity.'* 



In examining this section of the evidence which Mende- 

 lian analysis has provided we meet three facts of surprising 

 novelty. First, that the relations of yellow to black and 

 other colours may be entirely different in animals so nearly 

 related as the rabbit and the mouse. Secondly, that in mice 

 yellows never breed true, though in the rabbit, for instance, 

 no such difficulty occurs. Thirdly, that in rabbits the pre- 

 sence of the factor for the grey or agouti type of coloration 

 alters the appearance of the yellow animal, giving it a white 

 belly and tail, whereas yellows destitute of this factor have 

 these parts of a bluish colour. Such facts cannot of course 

 be properly interpreted until the chemistry of pigmentation 

 is more fully understood, but taken together they ilkistratc 

 the extraordinary multiplicity of specific rules which genetic 

 research reveals. 



8—2 



