644 MURID^— MUS 



up the dark pigments of its segment in turn gradually oxidise and 

 become yellow. In some "voles," &.%., Microtus orcadensis (seep. 458 

 above), a similar process appears to take place occasionally. 



Extensive researches upon the inheritance of the coloration, coat 

 pattern, and of some physical defects in the House Mouse, have been 

 made by the Mendelians. The literature relating to this work has 

 become quite voluminous, and no more than a mere outline of the 

 broader results can be attempted here. The experiments have mostly 

 been made with tame "fancy" mice, but these have been crossed from 

 time to time with wild animals. The various conditions investigated 

 have been proved for the most part to depend upon the presence or 

 absence of certain definite factors, and to obey Mendel's law in 

 inheritance. In the following paragraphs the capital initial signifies 

 the presence of a factor as opposed to its absence, denoted by a small 

 initial ; in general, X is dominant, x recessive. 



As regards pigmentation, a considerable number of factors appear 

 to be involved. Colour is produced by a special factor, C, and if this be 

 absent (c) the mouse will be an albino even if it carry all of the other 

 pigment factors. The depth of the coloration depends upon another 

 factor, D ; when this is present with C, the pigment granules are 

 developed in full number, and the colour is intense or saturate ; in its 

 absence {d), there are fewer granules and the colour is dilute. The 

 precise hue of the mouse depends upon a large number of factors, 

 known as " colour determiners " ; these determiners stand apparently 

 in a certain definite relation to each other. When all the normal 

 determiners are present, together with C and D, the animal is in 

 appearance an ordinary wild House Mouse, grey or " agouti " in colour. 

 Should the grey determiner (6^)^ be lacking, and the black (^) and 

 chocolate {Ch) determiners be both present, the mouse will be black — 

 the determiner B masking the determiner Ch. To be chocolate in 

 colour the mouse must not only carry Ch, but it must lack the 

 determiners G and B. Grey cannot, however, be called dominant to 

 black, because these factors belong to different allelomorphic pairs, 

 and Bateson, therefore, introduced the terms " epistatic" and " hypostatic" 

 to express the relationship of the different colours ; thus, in relation 

 to black {B), grey {G) is epistatic, while chocolate {CJi) is hypostatic. 



The combined researches of Cu6not, Durham, Castle and Little, 

 Hagedoorn, and others, have shown that yellow mice belong to two 

 entirely distinct groups, namely: — (i) that in which yellow arises 



' Strictly, there is no "grey determiner" at all, there being no grey pigment in 

 mice. The grey colour is produced by " barring and ticking," i.e., by the orderly 

 arrangement of the three pigments, black, brown, and yellow, in each hair in definite 

 bands of restricted extent. This arrangement is brought about, according to the 

 Mendelians, by the presence of a special factor (or pair of factors, according to 

 Hagedoorn) called the "grey determiner" above. 



