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, 7, which acts upon all the color factors. 

 According to Went worth 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 



PIG. 186. The skewbald 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, 



