DUNN — UNIT CHAEACTEB VARIATION IN RODENTS 
133 
YELLOW 
The self or solid yellow coat coloration in rodents appears to be 
divisible as to its cause into two different categories. In the first 
of these may be placed those yellow varieties which have arisen by a 
change in a gene governing the extension of black and brown 
to the fur, and the alternative (allelomorphic) condition of restriction 
of these melanic pigments to the eye, while the pelt is yellow. In the 
presence of this gene (restriction [r]) the melanic pigments are probably 
not produced in sufficient amount or to a sufficient intensity to invade 
the fur, leaving the residual yellow which is present in all ‘‘agouti’^ 
animals in possession of the whole extent of the hair. This gene 
is recessive to full extension and is distinct in its inheritance from the 
gene which determines the barring of each hair in the agouti pattern. 
Animals may possess the gene for yellow, with the gene for ‘‘agouti” 
or without it. “Agouti” yellows have much lighter bellies than non- 
agouti yellows. Restricted yellow occurs in the following rodents: 
Leporidce — Oryctolagus cuniculus. 
Muridce — *Microtus pennsylvanicus. 
Peromyscus maniculatus gambeli. 
Rattus rattus. 
*Rattus alexandrinus X R. rattus. 
CaviidoB — Cavia cobaya. 
The distinction between this yellow and the second type, about 
to be described, is made on grounds of the mode of inheritance only, 
for the appearance of the latter type is identical with that of the former. 
The pelt of the second type is yellow and the eyes are dark. The 
gene which differentiates it is however dominant over its allelomorphs 
agouti and non-agouti. This dominant yellow is known only in the 
house mouse and because of its peculiar mode of inheritance has been 
the subject of considerable research through a combination of genetic 
and embryological methods to which Castle, Little, Kirkham and others 
have contributed. Such investigations have established the following 
facts: 
(1) Yellow house mice do not breed true but when bred together 
always produce yellow and non-yellow young in the ratio of 2:1. 
(2) Litters from yellow by yellow are on the average 25 per cent 
smaller than litters from non-yellow varieties. 
(3) In the uteri of yellow females pregnant by yellow males there 
have been found disintegrating embryos approaching 25 per cent of 
the total embryos. 
