224 GENETICS IN RELATION TO AGRICULTURE 



pigment in the hairs thus producing the gray or agouti pattern. There 

 are variations in the regional distribution of the restrictive action. The 

 jillelomorphic condition a gives self-colored individuals. 



B the factor for black. The allelomorph 6 conditions a brown col- 

 oration instead of black. 



C the basic color factor in rodents. The allelomorphic condition 

 represented by c gives albinos. 



E a factor conditioning the extended type of pigmentation of self- 

 black or brown animals. The allelomorph e gives the black-eyed or 

 brown-eyed red or yellow coat. 



R the factor for rough or resetted coat, as distinguished from the 

 smooth coat determined by the allelomorph r. 



The work of a host of investigators has demonstrated beyond question 

 the Mendelian inheritance of these factors in races of the tame guinea- 

 pig. Castle in particular has demonstrated how these factors behave in 

 Mendelian fashion, one among the first investigations establishing the 

 general validity of Mendelian principles. Moreover, these conclusions 

 have been abundantly confirmed by investigations with other rodents, 

 which appear to possess a closely analogous series of color factors. 



Detlefsen's experiments were conducted by crossing tame female 

 guinea-pigs to wild males, and then mating back the hybrid females to 

 tame male guinea-pigs. This was necessary because the male hybrids 

 were sterile until back crosses to the tame guinea-pigs had been made for 

 two or three generations. Crossing back to the wild species was impos- 

 sible on account of the scarcity of wild animals and their failure to breed 

 freely under domestication. The investigations were carried through 

 eight generations, during which many types of matings were made, 

 and a total of 1160 hybrids were reared and studied. 



As a result of these investigations Detlefsen concludes that the wild 

 rufescens is of the constitution AABBCCEErr with respect to the factors 

 noted above. Moreover, the relation of these factors as respects domi- 

 nance and segregation was throughout identical with the relations 

 displayed in intervarietal crosses in the tame guinea-pig. Recombinations 

 of factors occurred in the normal fashion so that it was possible to secure 

 hybrids showing any type of coloration found in the tame guinea-pig. 

 The conclusion, therefore, that interspecific crosses between C. porcellus 

 and C. rufescens display complex Mendelian inheritance appears to be 

 established by these investigations. 



It may be pertinent, however, to enquire N whether homologous factors 

 normal for the two species are really identical. If we assume that the two 

 species possess similar genetic constitutions, i.e., have similar sets of 

 chromosomes bearing the factors in like arrangement, it is entirely con- 

 ceivable that, although the formal arrangement of factors in the heredi- 



