PART I. COLOR AND COAT CHARACTERS. 

 3. INTRODUCTORY DISCUSSION. 



Instances of alternative or Mendelian inheritance have been rapidly 

 accumulating since the rediscovery of Mendel's law in 1900, but most 

 of the cases known among mammals are based on relatively simple and 

 easily executed crosses, namely, crossing varieties of a species. Hence 

 the criticism has been offered that this form of inheritance does not 

 occur in species crosses or in nature. It has been maintained that 

 Mendelian phenomena are the result of laboratory methods, in which 

 we deal with man's domestic varieties. No contention is offered that 

 this or any other wild cavy mates with the guinea-pig in nature. We 

 have no evidence for or against such an hypothesis. In fact, it is more 

 probable that such crosses do not occur, for the repulsion which one 

 species of mammal usually shows to mating with another was evident 

 even in this experiment. When, however, a species cross is actually 

 made, whether it is in the laboratory or elsewhere, the data accruing 

 from the experiment may be legitimately offered to bear on the mode of 

 color inheritance in a species cross. 



The papers of Castle (1905, 1905a, 1907, 1907a, 1908, 1909) and 

 Sollas (1909) deal with the subject of color inheritance in guinea-pigs in 

 a summary manner, and so much has been written upon this subject in 

 other forms that I should feel most apologetic in offering more data 

 upon alternative inheritance of color in plants or animals were it not 

 for the fact that my observations cover a very definite category of 

 cases which have received little attention up to the present time, and 

 which may be of some general interest to students of heredity because 

 of the nature of the cross which gave rise to them. 



The symbols used to designate the color and coat factors are, briefly, 

 as follows: 



C, a factor necessary to the production of color in animals. Albinos 

 lack this factor; the allelomorphic condition is represented 



by c. 

 A, a factor restricting black or brown in the individual hairs, pro- 

 ducing the ticked or agouti type of coloration. This factor 

 may restrict differently in different parts of the coat. Black 

 and brown are restricted in the yellow subapical band on the 

 dorsal surface. They may be completely restricted on the 

 belly, giving yellow belly, as in the domestic guinea-pig; or 

 they may be partially restricted, and so allow a ticked or 

 barred appearance on the belly as well as on the back. The 

 latter is the condition in some wild Cavia rufescens and some 

 hybrids. The allelomorphic condition is designated by a. 



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