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 
allelomorphic 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 rosetted 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 AA BBCCEHErr 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 whether homologous factors 
normal for the two species are really identical. If we assume that the two 
species possess similar genetic constitutions, 7.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- 
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