THE HOUSE MOUSE 645 
by means of the suppression of the chocolate determiner (Cz); 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 (4) 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 C/% has been obtained. 
Much greater interest attaches to the second group of yellow mice. 
These were first studied by Cuénot, 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 (Z/) mice, although large numbers have been bred ; 
such yellows are always heterozygous (/z), and when mated together, 
as Castle and Little have shown, they produce yellow and non-yellow 
young in the ratio 2:1 instead of 3: 1,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. 
Cuénot, 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 wellknown. These facts afford strong grounds 
for the presumption that the introduction of the yellow determiner 
(J) 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 
