CHAPTER IV 



HEREDITY OF COLOUR. 



Factors determining Colours : the Ratio 9:3: 4. The 

 "Presence and Absence" Hypothesis. Epistatic and 

 Hypo static Factors Colours of Mice Pied Types 

 A Dominant Piebald. 



WITH regard to the application of the Mendelian system 

 to problems of colour inheritance the evidence is now 

 considerable. The fact that in both animals and plants 

 albinism behaved as a recessive to colours was soon dis- 

 covered. Several examples among plants are mentioned by 

 de Vries in his first paper on this subject, and shortly after 

 similar facts were recorded in regard to animals. Since in 

 the course of a large range of experiments with many species 

 of animals and plants no case to the contrary has been met 

 with, it may perhaps be asserted as a general truth that 

 pigmentation is always dominant to total absence of pig- 

 ment*. When however we proceed to the investigation of 

 the genetic properties of varieties which are so far deficient 

 in pigment as to be called sometimes partial albinos, we 

 find that various specific rules are followed and no uni- 

 formity of behaviour has yet been discovered. White fowls 

 for instance are thus commonly spoken of as partial albinos, 

 but the pigmentation of their eyes sharply distinguishes 

 them from albinos which are destitute of pigment, and 

 many of their genetic properties are found experimentally 

 to be quite distinct from those of real albinos. The same 

 is true of certain varieties of plants, which though varying 

 from the specific type by possessing white flowers have 

 yet some red or purple sap in the stem or elsewhere. With- 



* The Axolotl is perhaps an exception. See p. 43. The fact that in 

 plants colourless chromoplasts are dominant to yellow chromoplasts scarcely 

 constitutes an exception, for the yellow of the chromoplasts is not pigment 

 in the usual acceptation of that term. 



