Wright 



Color Inheritance in Mammals 



563 



not much affected, while the rest of the 

 coat shows an even mixture free from 

 the dappling of grays. There is no 

 change with age. The relations of 

 gray and roan have not been thoroughly 

 cleared up. Wentworth has given data 

 which show that roans may transmit 

 gray but that grays cannot transmit 

 roan. That is, horses of formula GgRr 

 are roans in the stud book. A priori 

 this seems a very remarkable result. 

 It appears to mean that the gray factor 

 which whitens animals which would 

 otherwise be solid colored is somehow 

 inhibited in the presence of another 

 factor with a whitening effect. The 

 explanation, however, is probably, as 

 Wilson suggests, that the real distinction 

 between roans and grays is not generally 

 recognized. A horse which is born 

 chestnut and turns gray is called a gray, 

 but one born a chestnut roan is called 

 a roan even though it later becomes 

 white, the latter effect being looked 

 upon as associated with the roan effect 

 in such cases. Thus roans should 

 probably be divided into two classes, 

 those which hold their color (ggRr) 

 and those which become gray (GgRr). 



Mendelian inheritance has been clearly 

 established in the case of the pattern 

 of skewbald and piebald horses. The 

 pattern appears to be quite similar to 

 the piebald patterns of rodents in char- 

 acter and so is classed with them here 

 in class lai. It is, however, a dominant. 

 Self by self, seems to give only self 

 (except for small markings), while pied 

 by self was found in a tabulation by 

 Waltherii to give 109 pied to 115 self. 

 This ratio indicates as one would expect 

 that practically all piebalds are hetero- 

 zygous. Solid white horses are occa- 

 sionally found, the white sometimes 

 even invading the eyes. Possibly these 

 may be homozygous for the piebald 

 factor. 



Horses with white markings on head 

 and feet are very common but do not 

 have the piebald factor. Walther could 

 find no simple mode of inheritance. 

 They seem to be comparable with the 

 Irish rats which Hurst found to be free 

 from the hooding factor. 



Bay is agreed by all writers to be a 

 simple dominant over black. A domi- 

 nant inhibitor of black is involved 

 which can best be compared with the 

 agouti factors of the rodents and is 

 therefore put in class 2a i. 



Brown is a color which has caused 

 much trouble. Sturtevant believed that 

 browns were generally heterozygotes be- 

 tween black and bay. Anderson con- 

 siders it to be a composite term at least 

 among saddle horses, including two 

 distinct varieties genetically, dark bays 

 and rusty blacks. Most browns appear 

 to have the bay factor. 



The factor by which chestnuts of all 

 shades differ from blacks and bays is 

 somewhat difficult to classify. An ordi- 

 nary chestnut shows little but red and 

 might be compared with the recessive 

 red of guinea-pigs. But liver chestnuts 

 show a larger amount of dark pigment 

 which, however, is brown, not black. 

 Thus the factor may well be placed 

 in class 2b with the chocolate factor 

 of rodents. Appearances suggest that 

 livers differ from ordinary chestnuts 

 in the same way that blacks do from 

 bays, /. e., in having factor a in place 

 of A, but the writer knows of no data 

 which bear on this point. 



GRADES OF INTENSITY 



There are many grades of intensity 

 in horses, especially noticeable in chest- 

 nuts, which remain to be analyzed. A 

 dominant general dilution factor by 

 which buckskin duns, mouse-colored 

 horses and creams differ from bays, 

 blacks and chestnuts seems best estab- 

 lished. Although dominant, it is put 

 for the present in class lb, the factors 

 of which it most resembles in effect. 

 Besides this, Wentworth has shown that 

 there is a recessive factor which is re- 

 sponsible for the difference between 

 light-maned sorrels and those with 

 dark manes, a result which has been 

 confirmed by McCann.^^ Light-maned 

 liver chestnuts which seem to have this 

 factor are not uncommon. It may also 



" Walther, Ad. 1913. Zeit. f. Ahst. u. I>r. 10:1. 

 J^McCann, L. P. Journal of Heredity, 7: 370-372. 



