THE HOUSE MOUSE 645 



by means of the suppression of the chocolate determiner {Ch) ; and 

 (2) that in which the yellow colour is due to the presence of a definite 

 yellow determiner. 



Yellow mice of the first type were studied by Hagedoorn, who 

 proved that all mice lacking the chocolate determiner are some shade 

 of yellow; that in such mice, if the determiners for grey {G) and 

 black {B) are present, the mouse appears as a " yellow agouti," while 

 it is "tortoise" if the black determiner alone is present. This group 

 is apparently strictly comparable with the yellow forms of rabbits 

 and cavies, in both of which, as here also, yellow shows itself to be 

 hypostatic to black and grey. Mice homozygous for ch are readily 

 produced when once a culture lacking Ch has been obtained. 



Much greater interest attaches to the second group of yellow mice. 

 These were first studied by Cu^not, and afterwards by Miss Durham, 

 Castle and Little, and still more recently by Hagedoorn. Here the 

 yellow colour is due to a yellow determiner, called / by Hagedoorn, 

 which shows itself to be epistatic to grey and black, and which is quite 

 unknown in wild House Mice. No one has so far succeeded in obtaining 

 homozygous yellow (//) mice, although large numbers have been bred ; 

 such yellows are always heterozygous (/?), and when mated together, 

 as Castle and Little have shown, they produce yellow and non-yellow 

 young in the ratio 2 : i instead of 3 : i, as would be expected by the 

 application of ordinary Mendelian principles. According to Castle 

 and Little it would seem that a whole class, viz., that of the homozygous 

 yellows (//), is absent from the progeny ; not because yellow ova fail 

 to be fertilised by yellow spermatozoa in due numbers, but because 

 the homozygous germs so produced perish soon after they are formed, 

 having apparently some physiological inability to develop further. 

 Cudnot, Miss Durham, and Castle and Little all found evidence (smaller 

 litters and a greater liability to sterility) of diminished fertility in 

 these yellow mice, while the frequent tendency of such animals to 

 become excessively fat is well known. These facts afYbrd strong grounds 

 for the presumption that the introduction of the yellow determiner 

 (/) gravely deranges the physiological equilibrium of the individuals 

 carrying it. The question as to how this strange determiner has been 

 introduced is quite unsettled ; the most plausible explanation yet 

 offered is that it has been brought into the breeds showing it by 

 means of hybridisation with some other species at present not identified. 

 Bateson, on the ground of a claim by a well-known breeder of mice 

 to have made such a cross, thought that there might have been a 

 cross with the Field Mouse. However improbable this view may be, 

 it cannot be dismissed without further experiment, because, apart 

 from the old statement made by Melchior, cited on p. 552 above, 

 Hagedoorn states that, although the species do not mate together 



