Chestnut "Fox" Friesians

The chestnut factor refers to the occasional birth of chestnut Friesian foals (also referred to in recent years as Fox Friesians). The Friesian breed has traditionally been black in coloration, and this has become the breed standard, so deviations from the standard, such as chestnut coloration is for the most part undesirable among breeders. The early Friesian horses had coat colors of all varieties, but since then these variations have been selectively bred out in favor of black. However, the chestnut gene is what is called 'recessive' meaning that it tags along with the black color gene, but doesn't show itself. We'll call the chestnut gene (c), small letter because it is recessive, and the black gene (B), large letter because it it dominant. So black horses whose dominant color gene is black, but who carry a recessive chestnut gene (Bc) may, when bred to another black horse who carries a recessive chestnut gene (Bc), produce a chestnut foal (cc). In order for a chestnut foal to be produced both parents have to be Bc, and the odds have to be in their favor, for even in this situation, the black gene is still dominant, allowing only a 25% chance of producing a pure chestnut foal (cc). If a horse carrying a chestnut gene (Bc) is bred to another who does not carry it (BB), the resulting foal has no chance of being chestnut. Obviously the same is true if both parents are (BB), in which case they really will be a true black.