168 Inheritance of Coat Colour in Mice 



considering, is to be suspected, and 1 doubt whctlier the observations 

 can be used either for or against the conclusion that the ratio of yellow 

 to non-yellow in F„ is 2 : 1. 



The non-viability of pure yellows raises an important physiological 

 question, but we have no indication as to what may be its cause. It 

 should be remembered that the mortality may, for aught we yet know, 

 occur at any age between fertilization and maturity. 



In the report to the Evolution Committee(3), I have already stated, 

 that the pigments of the eye of the yellow mouse may be black or 

 chocohite but never yellow. If the yellow mouse throws chocolate 

 young but never black the eye will be found to be pigmented with 

 chocolate, often chocolate pigment will also be found in the hairs of this 

 animal. 



A yellow mouse which throws black young will have black pigment 

 in the eyes and some black pigment will always be found in the hair. I 

 have never found black pigment in the hair of a mouse with chocolate 

 only in the eyes. 



I have examined several hundred yellow mice and never found an 

 exception to this statement. 



The hair and the eyes are a key to the genetic behaviour, or one 

 may equally well say the genetic behaviour is the key to the pigments 

 of the hair and eyes of the yellow mouse. Both black and chocolate 

 pigments will be found in the eyes of the yellow mouse with agouti 

 determiner. 



Yellow mice are subject to an abnormal development of fat in their 

 tissues. All the fat depots become loaded to an extraordinary degree. 

 This development of fat renders them unable to breed. It is a well- 

 known fact to the breeders of Fancy mice. 



The question of dilution is also a difficulty in yellow mice. Yellow 

 mice vary very much in their colouring. Some are very deep yellow, 

 some much paler, some are deeply coloured dorsally and very light 

 underneath, pale almost to whiteness. I do not mean piebald, but the 

 colour fades off gradually to a very pale cream. The result is that it is 

 very difficult and often impossible to decide whether a mouse belongs 

 to the dilute variety or not. Of course many mice are so pale all 

 over, one would not hesitate to class them as dilute yellows, that is 

 creams. But there is a very large section whose classification can only 

 be determined by their genetic behaviour. To illustrate the difficulty 

 I will mention the case of two mice which I bred together and classed 



