COLOR INHERITANCE IN MAMMALS 



VII, The Horse — Studbooks Have Afforded Data for Many Investigators— Many 



Different Pairs of Allelomorphs to be Found in All Combinations 



— Some Interesting Cases in Hybrids 



Sewall Wright 

 Bureau of Animal Industry, Washington, D. C. 



HURST 1 showed as early as 1906 

 that chestnut differs from the 

 darker colors by a simple Men- 

 delian recessive and this result 

 has been confirmed by all later workers. 

 Most of the remaining factors were 

 rapidly worked out by Wilson,- Sturte- 

 vant,^ Anderson/ Wentworth,^ and 

 others with results largely in harmony. 

 There still exists, however, one very 

 important difference of opinion. Most 

 of the factors were worked out simply 

 in opposed pairs of characters and little 

 attention has been paid to the relations 

 of the different pairs to each other. 

 Two different theories as to these re- 

 lations have arisen. The present paper 

 follows in the main the view very clearly 

 presented by Wentworth and probably 

 most generally held, viz., that there are 

 many different pairs of allelomorphs 



B.\Y-HORSE — GR-WMAB 



lai 



la2 

 las 

 lb 



2a, 



2a2 

 2a3 

 2b 



G, 8 

 R, 1- 



V, V 



D, d 

 M, m 

 A, a 



B, b 



G — gray 

 R — roan 

 V — piebald 



D— dun 

 hhmm — sorrel 

 a — black 



h — chestnut 



Classification explained in paper on 

 the mouse, Journal of Heredity, 8:373, 

 August, 1917. 



which may be found in all combinations. 

 Wilson,*' on the other hand, holds that 

 most of the factors belong to the same 

 series of allelomorphs. Thus he con- 

 siders gray, dun, bay, black and chest- 

 nut to be such a series of what he calls 

 polygamous factors. This series is of 

 a very different kind from the nine sets 

 of multiple allelomorphs known among 

 rodents. In the latter, the factors 

 produce a graded series of effects when 

 arranged in the order of dominance, a 

 fact which suggests that in these cases 

 we are simply dealing with different 

 levels of potency of some one genetic 

 as well as physiological factor. Wilson's 

 series cannot possibly be arranged in 

 such a way. It is difficult to conceive 

 how variations in one physiological 

 factor could produce such diverse ef- 

 fects. The series, may, indeed, be com- 

 pared with Nabour's^ series of color 

 mutation in grasshoppers and perhaps 

 with the condition of complete linkage 

 found in factors of the same chromo- 

 some in the male Drosophila,^ or the 

 female silkworm.^ If Wilson's hypoth- 

 esis is correct, it is easiest to suppose 

 that the factors are related merely in 

 a mechanical manner in the germ-cells 

 and not as variations of the same factor. 



FACTORS APPEAR INDEPENDENT 



However, there is considerable evi- 

 dence in the horse which appears to the 

 writer to favor decidedly the hypoth- 



1 Hurst, C.C. 1906. Pror. Rov Soc.,11 -Md,. 



2 Wilson, J. 1010. Sci. Proc. Dub. Soc, 12:331. 



'Sturtevant, A. H. 1910. Biol. Bull., 19:204; 1912. Jour. Gen. 2:41. 



^Anderson, W. S. 1913-1914. Amer. Nat., 47; Kv. Sta. Bull., 180:121. 



6 Wentworth, E. N. 1914. Zeit.f. Absl. u. Ver., 11:10. 



8 Wilson, J. 1916. A Manual of Mendelism. 152 pp. 



^Nabours, R. K. 1914. Jour. Gen., 3. 



* Morgan, Sturtcvant, Muller and Bridges, 1915. The Mechanism of Mendelian Hereditv. 



^ Tanaka, Y. 1914. Trans. Sapporo Nat. Hist. Soc. 5. 



561 



