134 
JOURNAL OF MAMMALOGY 
Ordinary heterozygotes (hybrids in one character) when bred together 
produce 25 per cent pure dominants, 50 per cent heterozygotes, and 
25 per cent pure recessives. In the offspring of yellow mice the two 
latter classes have appeared in the expected proportions; pure dominant 
yellows have never been disclosed by breeding tests. It has there- 
fore been concluded that the 25 per cent missing from the litters, the 
25 per cent of disintegrating embryos, and the 25 per cent of expected 
pure yellows are the same. The intrauterine death of this class has 
been supposed to be due to a recessive lethal gene which when received 
from both parents causes the death of the resulting zygote or individual. 
In every case this lethal gene has been transmitted with the gene for 
yellow. It may be either completely linked or identical with the gene 
for yellow. At any rate it is present at the same locus with the gene 
which determines yellow, and any individual which receives yellow from 
both parents receives likewise the lethal gene from both parents and 
is doomed to death before birth. As to why this combination of two 
lethal genes is fatal we are still in the dark. 
This yellow gene and the lethal associated with it are known only 
in house mice,® and the restricted yellow of the other species has not 
been reported in house mice. They are probably not homologous 
variations in spite of their similarity in appearance. The ‘‘yellow’’ 
varieties of rats are not really to be classified with other yellow rodents 
since they are actually ‘^agoutis”, differentiated from the wild gray by 
the pink-eye gene (to which we have referred) or by the very similar 
red-eye gene which act selectively on the melanic pigments to reduce 
rather than restrict them. 
WHITE-SPOTTING 
Almost as common as albinism among rodents is the spotting of cer- 
tain portions of the coat with white. The white areas are as devoid 
of pigment as in albinos but here the likeness ends. Genetically white- 
spotting and albinism are distinct and contrary to the popular belief 
are not quantitatively but qualitatively unlike. Albinism is funda- 
mentally the loss or change of a factor for the development of a per- 
oxidase essential to the production of any pigment (cf. Wright ’17) 
and its effects are of a general nature throughout the pelt and eyes. 
Spotting appears to be a change in a factor governing the distribution 
® Several other factors may modify the appearance of yellow in mice; for 
instance, certain darkening factors in the presence of the yellow gene produce 
the black-and-tan and sable varieties of mice, while intensifying factors in the 
presence of yellow produce the brighter orange or red varieties. 
