118 INHERITANCE IN GUINEA-PIGS. 



SUMMARY. 



The principal results which have been reached may be summarized 

 as follows: 



1. A classification of guinea-pig fur, skin, and eye colors is given with 

 definitions of fur colors in terms of Ridgway's charts (1912). 



2. Rodent color factors are conveniently classified as follows : 



a. Factors which affect the distribution and intensity of color largely irrespective 



of the kind of color. 



b. Factors which govern the differentiation between yellow and dark colors in 



colored areas of the fur. 



c. Factors which determine the kind of dark color in the areas with dark pigmenta- 



tion in fur and eyes, without influence on yellow areas. 



Definitions of all known guinea-pig color factors are given on this 

 basis and a table of the color varieties arising from combinations of 

 these factors is given. 



3. Genetic and biochemical evidence on the physiology of pigment 

 formation suggests the hypothesis that the three groups of factors 

 determine respectively the distribution and rate of production by the 

 nucleus of the following substances : 



o. A peroxidase which, acting alone, oxidizes chromogen in the cytoplasm to a 

 yellow pigment but is so unstable that it must be produced at a relatively 

 high rate to give any pigment at all. 



b. A supplementary substance which, united with the first, makes it a dark-pig- 

 ment-producing enzyme and of such stability that color develops at a 

 much lower level of production of peroxidase than when the supplement 

 is absent. Above the level at which both produce effects, the dark and 

 yellow-producing enzymes compete in the oxidation of chromogen. 



e. Additions to the second substance which cause variations in dark color but not 

 in yellow or in the competition between dark color and yellow. 



4. There is a continuous series of variations in intensity of pigmen- 

 tation hi the yellow, brown, and black series and in eye color. The 

 ordinary dilute guinea-pigs are found to be imperfect albinos in the 

 sense that dilution is due primarily to a member of the series of allelo- 

 morphs intensity, dark-eyed dilution, red-eyed dilution, and albinism, 

 with dominance in the order of increasing intensity. 



5. A further step in the analysis of the continuous series of variations 

 of intensity is taken hi the demonstration that dilution is imperfectly 

 dominant over red-eye and albinism as regards the yellow series of 

 colors, and that dilution and red-eye are imperfectly dominant over 

 albinism, as regards the black series. Smaller effects are due to the 

 residual heredity of different stocks and to age. 



6. Evidence is presented which confirms the hypothesis of Detlefsen 

 (1914) that the light-bellied agouti pattern of tame guinea-pigs, the 

 ticked-bellied agouti of hybrids between the tame guinea-pig and Cavia 

 rufescens, and non-agouti (as seen in self blacks or browns) form a 

 series of triple allelomorphs in which light-belly is the highest dominant 

 and non-agouti the lowest recessive. Evidence is presented which 



