Wright: Color Inheritance in Mammals 



231 



slightly dilutes eye color. Finally factor 

 b of class 2b further modifies the sepia 

 in the coat and eye producing light 

 brown but does not affect the yellow. 

 The animal is a brown-eyed light brown 

 agouti with yellowing ticking and a 

 white pattern in the coat. 



It is often difficult for one who is not 

 working in the inheritance of coat color 

 to understand just what color is sup- 

 posed to be determined by a given array 

 of factors. Probably this can be done 

 most easily by considering the factors in 

 the order just given. First are the 

 factors of class la which determine 

 patterns of color and white. No factors 

 considered later can change these white 

 areas. Next to be considered is the 

 pattern of dark and yellow colors visible 

 in the colored areas. These are deter- 

 mined by class 2a. Finally the kind of 

 yellow in the yellow areas may be seen 

 by noting the factors of class lb and 

 the kind of dark color in the remaining 

 areas by a simultaneous consideration 

 of classes lb and 2b. Eye color is 

 generally determined wholly by these 

 last two classes, but occasionally ex- 

 treme white ])atterns of class la invade 

 the eye. 



DISCUSSION OF CLASSES OF FACTORS^^ 



Class la. — White patterns are very 

 common in mammals and most of them 

 are obviously determined by factors of 

 this class even where the mode of in- 

 heritance has not yet been thoroughly 

 analyzed. The factors which determine 

 the white face of the red Hereford 

 cattle are a good example. The same 

 white face appears in the black cattle 

 from the cross of Hereford with Aber- 

 deen-Angus. Evidently the factors in- 

 volved strike at color in general regard- 

 less of its quality. The same is true of 

 the different types of white patterns 

 found in roan and in white Shorthorns, 

 in the black and white Holsteins, and in 

 Dutch belted cattle. The common 



white patterns of horses, dogs and cats 

 are similarly independent of the ground 

 colors of the animals. This seems to be 

 true of the belt in Hampshire hogs, but 

 is not so certain in other white patterns 

 in hogs which may correspond to 

 extreme dilution of yellow patterns. 

 One or more recessive Mendelian factors 

 have been demonstrated for the patterns 

 of piebald mice, hooded rats, and Dutch 

 rabbits, while a dominant factor is 

 responsible for the English pattern of 

 rabbits and the white blaze in man. 



The mechanisin by which such pat- 

 terns are determined is interesting to 

 speculate upon, but very little is yet 

 known. In the case of the English 

 rabbit, Onslow has demonstrated the 

 presence of an inhibitor which prevents 

 the oxidation of tyrosin by tyrosinase. 

 Apparently the power of a cell to 

 produce this inhibitor is determined by 

 a variety of conditions of which the 

 level of the English factor (absent, 

 heterozygous or homozygous) is one and 

 differences brought about in regional 

 differentiation another' As a result of 

 the combined effects of these conditions 

 a given cell either has no power to 

 produce the inhibitor or can produce 

 sufficient to inhibit any intensity of 

 color. The extent of white patterns 

 seems to be the same in general whether 

 the ground color is intense or dilute. 

 In Dutch rabbits Onslow found no 

 enzyme inhibitor but simply an absence 

 of jDeroxidase. Here we must suppose 

 that some essential condition in the cells 

 for production of enzyme I is deter- 

 mined by the array of recessive white 

 pattern factors in conjunction with 

 regional differentiation. 



The maltese type of dilution which 

 appears under the microscope in such 

 a case as the blue rabbit as due to an 

 alternation of colorless sjmces with 

 intense pigmentation within each hair 

 is put provisionally in class la. Reces- 

 sive factors which determine simultane- 



^*Most of the statements in regard to color inheritance are based on well-known investiga- 

 tions. A very detailed review of the literature to 1913 is given by Lang, 1914 {ExperimentelJe 

 Vererbungslehre, pp. 467-888). A discussion of our present knowledge on the .subject with ex- 

 tensive bibliography is given by Castle, 1916 (Genetics and Eugenics). The writer may say 

 that all of the statements in regard to guinea-pigs can be based on his own experience, and he 

 has also had the opportunity of becoming directly acquainted with the mode of inheritance of 

 most of the color varieties among rats, mice and rabbits as an assistant in Professor Castle's 

 laboratory for three years. Only a few references are cited in this paper. 



