Journal of Mammalogy 
Published Quarterly by the American Society of Mammalogists 
VoL. 2 AUGUST, 1921 No. 3 
UNIT CHARACTER VARIATION IN RODENTS^ 
By L. C. Dunn 
Recent progress in the study of the inheritance of coat colors in several 
species of rodents has revealed a rather striking similarity in the varia- 
tions which have arisen in distinct species of that order. This similar- 
ity is not only a matter of appearance, which is familiar to all students 
of mammals, but extends as well to the manner of inheritance, and most 
recently has been found to characterize the localization of the deter- 
minants or genes for similar variations in two species. Such identity 
of cause of the same variation in two or more species indicates that such 
variations are homologous, and that the species which give rise to them 
have a relationship of a somewhat different and more intimate kind 
than that implied in the theory of relationship by common descent. 
Before detailing the conditions in the species of rodents which have 
been studied, some explanation of the evidence and reasoning which 
underlie the localization of genes is due to the general reader. It is 
probably recognized by all students of biology that heritable variations 
arising generally by mutation are transmitted to the offspring in ac- 
cordance with certain definite rules, known familiarly as Mendeks 
laws of inheritance. The chief of these laws states that heritable 
characters are transmitted as discrete units which segregate in the 
formation of the germ cells. A second principle asserts that the segre- 
^ In this paper, which is to regarded as a cursory survey leading to a consider- 
ation of one or two special points rather than as an authoritative exposition of 
variation in rodents, I have not felt it essential to furnish a detailed bibliography. 
The necessary references may be found in Castle (1920) and Morgan (1919) as 
noted in the bibliography, and an excellent survey in Wright (1917), which also con- 
siders the physiological and chemical aspects of color variation and inheritance. 
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