DUNN— UNIT CHARACTER VARIATION IN RODENTS 
137 
Specimens of individuals of other species which are much darker 
than the wild type are often seen in museums, usually labelled ‘‘melano” 
or ‘^melanic variation.’’ Many of these should not, I beheve, be as- 
sumed to represent the. true black variation. Some, as in ‘‘melanic” 
squirrels of various species, prove on close examination to be only very 
much .darkened ‘‘agoutis” in which the black portions of each hair 
have been extended at the expense of the yellow portions. This 
variation has been studied by Punnett in the rabbit and found to be 
due to a gene distinct from black which is allelomorphic with the ex- 
tension-restriction pair of allelomorphs. Much darkened “agoutis” 
have been produced in mice by crossing intense blacks or black-and- 
tans (a darkened form of yellow) with wild agouti-colored mice. The 
darkness is due in this case to a series of modifying genes distinct from 
either black, “agouti” or yellow which in the presence of these genes 
bring about an increase in the amount and intensity of the black pig- 
ment granules and a reduction in yellow. Such extended or darkened 
“agoutis” can usually (though not always) be distinguished from black 
by the lighter belly which is typical of the “agouti” pattern. 
In addition to the coat color and pattern variations discussed above, 
many others have occurred in rodents which have been bred in the lab- 
oratory, though they are as yet known in too few species to make com- 
parisons profitable. One of these, dilution, is a unit character in mice 
and rabbits. In this variation, the pigment granules are clumped 
and reduced in distribution, producing when acting on black varieties 
the famihar maltese color of blue rabbits and mice. The maltese cat 
is the result of a similar variation from black. Dilution is a simple 
Mendelian recessive to full color. The red and black blotching of 
guinea-pigs, the ticking or banding of the belly hairs in guinea-pigs, 
and the white bellies of a fancy variety of “agouti” mice are also known 
to be Mendelian unit characters.^ 
SUMMARY 
A summary of the preceding discussion shows that we have examined 
five of the commonest variations in rodents : albinism, pink-eye, yellow, 
white-spotting, and black. In all of these the inheritance is known for 
at least three species, and in general the variant is recessive to the wild 
type. Dominant variations have occurred often enough (such as 
8 For a fuller discussion of these the interested reader is referred to Chapter 
XII in Castle’s “Genetics and Eugenics,” 1920, and to the series of papers by 
Wright cited above. 
