468 GENETICS IN RELATION TO AGRICULTURE 
This completes the formulation, so far as our present knowledge goes, 
for the series of colors, usually met with among horses, but there are a few 
others more rare and less in favor, the position of which from a genetic 
standpoint is almost wholly speculative. There appears to be, however, 
a dominant dilution factor, J, which acts upon all the color factors. 
According to Wentworth black with this factor becomes mouse colored, 
bay becomes dun of the particular shade known as buckskin, chestnut 
becomes yellowish dun, and sorrel with lighter mane and tail becomes 
cream colored with lighter mane and tail. The evidence, however, is 
by no means extensive enough to be conclusive and should receive 
Fig. 186.—The skewhald Iceland pony, Tundra, her skewbald filly, Circus Girl, by a 
bay Shetland pony, and her hybrid foal, Sir John, by the Burchell zebra, Matopo. (After 
Ewart.) 
experimental verification. There seems, also, to be a white which is 
distinct from the faded gray of old horses. This white is dominant to any 
color. Castle considers it an extreme extension toward white of the 
spotted condition. The types of spotting are mostly dominant over 
uniform coloration, and often the pattern is very faithfully reproduced. 
This statement applies to stars, blazes, skewbald markings, calico types 
of pattern, and other kinds of white spotting of the same general type. 
Fig. 186 shows a case of accurate reproduction of skewbald markings by 
the offspring of an Iceland pony when bred to a bay Shetland pony 
stallion. Tundra had previously produced a dun foal to the service of a 
stallion of unknown coat color, and subsequently she produced another 
skewbald foal. Her zebra hybrid foal, however, was of a dun color, 
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