Varietäten, Descendenz, Etybriden. 7 



Miss Durham points out that the allelomorphs to whieh the 

 various coat-colours are due may be represented thus: 



G. g. Presence and absence of the factor which gives the "agouti" 

 or "grey" pattern in the hairs. 



B. b. Presence and absence of the black determiner. 



C. c. Presence and absence of colour. 



If C is present without G or B the colour is chocolate. All albino 

 mice are to be represented as those from which C, i.e. the colour 

 chocolate is absent. 



The above factors give the following combinations: 



CGB Grey or Agouti. 



CgB Black. 



CGb Cinnamon Agouti. 



Cgb Chocolate. 



In addition D and d may be used to represent the presence and 

 absence of a factor which causes the dense deposit of pigment; thus, 

 CgBD is black, CgBd is blue; CgbD is chocolate, Cgdb is dilute 

 chocolate, or "silver fawn". 



When the pied condition is dominant, there is evidently another 

 factor P present, which inhibits colour in varying degrees, whereas 

 the recessive pied conditions are due to the absence of S, self-colour. 



Cuenot originally suggested that G should be taken as allelo- 

 morphic to B. Miss Durham points out that if G and B are allelo- 

 morphic, not to each other, but to g and b, their absences respecti- 

 vely, then black should appear in F 2 . 



Miss Durham made the experiment and obtained 



22 ag., 6 cinn. ag., 5 black, 3 chocolate; 

 the expectation being 



205, 675, 675, 2 25. 



Cuenot similarly bred 41 ag. , 15 cinn. ag., 15 black, 5 choco- 

 late (expectation being 4275 ag. , 14'25 cinn. ag., 14'25 black, 475 

 chocolate). Cuenot speaks of the appearance of blacks as unexpected 

 and attempts to account for them as being a dense form of choco- 

 late. Miss Durham shows that blue, not chocolate, is the dilute form 

 of black, while chocolate has its own dilute form, silver fawn. 



In the dark-eyed mice the inter-relations of the colours , with 

 the exception of yellow, are now clear, agouti, black, and choco- 

 late forming a descending series in which black is epistatic to 

 chocolate, and agouti to both. Miss Durham's results agree with 

 those of Cuenot and others in making it probable that yellows are 

 always heterozygous, but various difficulties are encountered when 

 a more precise Statement is attempted. 



Eye-colour. In dark-eyed mice, the eyes seen in situ are 

 apparently black, but when they are removed differences can be 

 seen at once, which are even more fully apparent when sections 

 of the eyes are examined microscopically. Miss Durham finds that 

 apparently all mice, which have black pigment present in the coat, 

 have black pigment in the eye, while those mice in which black 

 pigment is absent from the coat have only chocolate pigment in the 

 ej^e. Thus agouti and its dilute forms, and black and blue have 

 black eyes, while cinnamon agouti and its dilute forms, and cho- 

 colate and silver-fawn all have chocolate eyes. 



The clear yellow mice examined by the author have chocolate 

 eyes, as stated by Castle; but the author finds that the sooty-yellow 

 mice have black pigment in the eyes. 



In the three cases of ruby^es encountered by Miss Durham, 



