8 REVERSION IN GUINEA-PIGS AND ITS EXPLANATION. 



SUMMARY. 



1. The agouti coat characteristic of wild cavies and of most other wild 

 rodents is dependent upon the presence in the fur of black pigment disposed 

 in a definite pattern with red (or yellow). 



2. The factors which control, respectively, the development of black pig- 

 ment and the production of the agouti pattern are independent of each other. 



3. The agouti coat is obtained only when both these factors are possessed 

 by an individual. 



4. Only such agouti individuals as are homozygous in both factors breed 

 true under all circumstances. 



5. An agouti animal which is homozygous in A (the agouti factor), but 

 heterozygous in B (black pigmentation), may produce agouti young and red 

 ones, but not black. 



6. An agouti animal which is homozygous in B but heterozygous in A 

 may produce agouti young and black ones, but not red. 



7. An agouti animal heterozygous in both A and B may produce three 

 sorts of yoimg, agouti, red, and black. All Fi (reversionary) agoutis produced 

 by crossing black with red are of this sort. Agoutis of the other three sorts are 

 obtained only in the second or later generations of agouti young. 



BIBLIOGRAPHY. 



Castle, W. E., 1905. Heredity of coat-characters in guinea-pigs and rabbits. Carnegie 

 Institution of Washington PubUcation No. 23. 



1907. On a case of reversion induced by cross-breeding and its fixation. Science, 



N. S., vol. 25, p. 151. 



1907a. The production and fixation of new breeds. Proc. Am. Breeders' Associ- 

 ation, vol. 3, p. 34. 



Table 1. — Differences among red guineO'pigs as regards the transmission of agouti. 



