The Genetic Factors in tlie Development of the Housenaouse, etc. 107 



The third one, a male, N" 41 16, was mated to two silverfawns 

 (ABcdEFgH) females. He produced in three litters: 



d No 4116 X 2 Q ABcdEFgH 14 young, all silverfawn, ABcdEFgH, 

 thus showing to be homozygous for the absence of (", his formula 



being AABBccddeeFFggnU. 



If I add together the number of young produced in the litters 

 from one Co and one cc parent, I get 298 Cc animals and 281 cc ones, 

 the calculated proportion being: 289.5 : 289.5. 



In rabbits, the factor c has onlj' been lost some eight or nine 

 years ago, at least the first record of chocolate rabbits are that they 

 were bred in a second generation of a cross between a brown-nosed 

 himalayan (a possible sport) and a black one, in the south of Holland. 



The only combinations without c in rabbits which I have ever 

 bred or seen are cinnamon (ABcDEFGH), chocolate (ABcDEFgH), orange 

 (AbcDEFgH), and silverfawn (ABcdEFgH). 



N"s I, 4, 6, 10, II, 12, 16, 22 and 24 on the plate do not con- 

 tain c, all the others have it. 



Factor D. 



This factor has first been studied by Miss Durham. It is one 

 of the factors necessary to obtain a full pigmentation. The colours 

 whicli are produced without its cooperation have been called dilute. 

 Thus it may be said that blue (ABCdEFgH), N« 3 is the dilute form 

 of black (ABCDEFgH) No 15, that ABcDEFgH, No 4 (silverfawn) is dilute 

 chocolate (ABcDEFgH) No 16, that ABCdEFGH (blue agouti) No 14 is 

 dilute agouti, agouti being abcdefgh. No 13. 



But it is impossible to invent names for the hundreds of possible 

 combinations of genetic factors which are visibly different and 

 moreover rather useless. 



In some cases, the presence or absence of D may cause a much 

 greater difference than in others, so, for instance, is it nearly im- 

 possible to distinguish AhCDeFGh and AbCdei'"t;h, although nobod}' would 

 have any difficulty in telling ABCdEFgH (blue) (No 3) and ABCDEFgH 

 (black, No 15) apart, or an ABcdEFgii (silverfawn. No 4) and an 

 ABcDEFgH (chocolate. No 16). 



In some combinations of factors, tlie absence of D may affect 

 the colour of the eye, which becomes ruby under a certain light. 

 Thus are most dilute agouti (ABCdEFGH) N» 14, mice, and were all 

 my ABCdEFtiH cavies ruby-eyed. This colour of the eye is quite 

 different from that which is found in animals without A or without 



