226 GENETICS IN RELATION TO AGRICULTURE 



are produced, those of the genetic constitutions AA' and Aa respectively. 

 Phcnotypically these two classes of individuals are exactly alike for the 

 powerful tame agouti factor is alike dominant to the wild agouti factor 

 A' and to the tame non-agouti factor a. When the individuals of this 

 population were bred to tame non-agouti animals of the genetic consti- 

 tution aa the existence of the two above-mentioned genotypes was clearly 

 demonstrated, for half the individuals gave progenies exhibiting sharp 

 segregation into light tame agoutis and dark wild agoutis in approximately 

 equal numbers and the other half gave progenies consisting of approxi- 

 mately equal numbers of light tame agoutis and non-agoutis. Other 

 tests satisfactorily supported this analysis so that it may be concluded 

 that the agouti factor of the wild C. rufescens is different from the agouti 

 factor of the tame C. porcellus, but that they are allelomorphic to each 

 other. If we consistently follow up the hypothesis which we have devel- 

 oped as to the constitution of the hereditary material and the operation 

 of the chromosome mechanism, this can only mean that the factors for 

 agouti, although different, occupy corresponding loci in the hereditary 

 system of these two species. Aside from certain observations indicating 

 differences between the rough factor of wild and tame guinea-pigs we 

 have no evidence as to whether or not those other factors, the inheritance 

 of which was investigated, are different, but we may safely conclude 

 that factors of corresponding behavior occurred at exactly the same loci 

 in the hereditary system of the wild C. rufescens. 



Evidence as to the difference between the two agouti factors is also 

 provided by the irregular behavior of the wild agouti factor in the hybrids. 

 Although the first hybrids between the wild agouti and tame non-agouti 

 guinea-pigs are mostly of the dark wild type with ticked bellies in sub- 

 sequent generations there appear agoutis which are so light as to approach 

 closely the light agouti type of the tame parent and others are so dark 

 that the individuals show only a slight sprinkling of agouti hairs. Indi- 

 viduals displaying such modifications of the wild agouti pattern show no 

 very regular type of behavior, for dark individuals sometimes produce 

 some light individuals and the light individuals sometimes produce 

 some dark ones. The dark modification, however, is most common 

 and often becomes more pronounced upon successive dilutions with 

 tame blood. An interpretation of such phenomena cannot be made 

 satisfactorily unless we consider the agouti factor as a member of a com- 

 plex system of factors which together operate to give the agouti type 

 of coloration. From this standpoint it is not at all strange that the 

 wild agouti factor acting in conjunction with a corresponding system 

 of factors mostly derived from the tame guinea-pig should exhibit the 

 full power of its customary restrictive action because of a failure to set 

 up wholly harmonious relations with these factors. This modifying 



