42 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



an albino male guinea-pig which has spotted ancestry, and she gave 2 

 spotted and 2 self-colored young, and possibly a third spotted young 

 in a case of doubtful motherhood. The other, 9 75, bred to self- 

 colored males gave 4 self-colored young. Four other ^ wild females 

 (9 63, 9 68, 9 69, 9 253) were bred to brindled or spotted male guinea- 

 pigs, but their 40 offspring were self-colored. It would, therefore, 

 appear that the self-pattern of the wild and the f wild was dominant 

 to spotting. 



When the I wild females, which we know had a spotted father, were 

 mated to a pure race of spotted guinea-pigs, they produced 28 self- 

 colored and 18 spotted young. If dominant self-color and recessive 

 spotting were clearly allelomorphic, then we should expect an approxi- 

 mate equality. There is an excess of self-colored young, nevertheless 

 the spotted variety of hybrids was produced by the admixture of 

 spotting from the guinea-pig source. The clear dominance of the wild 

 self-pattern and that of the | wild was lost in the later generations 

 when the hybrids were continually mated back to the guinea-pig. In 

 this and other respects these later hybrids resemble the guinea-pig 

 itself, for dominance of self over spotting is incomplete in pure guinea- 

 pig races. Both brindled and spotted varieties of hybrids were pro- 

 duced as early as in the | wild, the F2 generation. 



INTENSITY AND DILUTION. 



In rabbits and mice, a dilute condition of pigmentation is known. 

 This condition is hypostatic to the ordinary intense pigmentation. 

 Black, brown, and red become ''blue," light brown, and cream, respec- 

 tively, when the dilute condition is present. This condition in guinea- 

 pigs is a distinct recessive factor, for if a cream and a blue are mated, 

 the offspring are blue; but if a red and a blue or a cream and black are 

 mated, only black offspring result. Dilute-pigmented guinea-pigs, 

 mated inter se, breed very true. Whether or not the intense and dilute 

 conditions in guinea-pigs are allelomorphic to each other is a difficult 

 question, but apparently they are. 



In the different races of hybrids, dilute animals have appeared. No 

 complete study of such hybrids has been made, for the number of 

 reliable cases is small, yet the fact that such dilute hybrids can occur, 

 just as in the guinea-pig, is certain. An apparent complication has 

 arisen in the case of the hybrids. As has been stated in the discussion 

 of the inheritance of black, there have occurred extremely dilute forms 

 which were not expected. The same is true of brown and cream. No 

 reason for the appearance of these very dilute hybrids can be assigned. 

 They are as light as any which have been obtained in guinea-pigs by 

 continued selection. Curiously enough, most dilute hybrids have 

 appeared when a particular strain of guinea-pig sires was used. These 



