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 is 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.
So? Well, because many horses carry the chestnut gene and it cannot be completely eradicated, the FPS registry has tested all its approved stallions for the gene so that mare owners can make informed decisions when breeding. The registry has also begun testing young stallions for the chestnut gene and pulling them from the approval process if they are carriers to reduce future incidences of chestnut foals. So a mare owner today can test their own horse to see if she carries the gene, then, if she does, then choose a stallion who is not a carrier, thus ensuring a black foal. Although the occurrence of chestnut foals is in no way a very serious breeding or health issue, since no actual physical harm comes to the foals and the only objection is cosmetic, it is however, something to be aware of for those in the FPS registry when choosing stallions for certain mares.
The horse pictured is a chestnut Friesian gelding owned by Adel Phillips. For more photos of chestnut Friesians, try visiting this website.
[The More Detailed Look at the Chestnut Factor] – An article by Dr. Geurts, reprinted from International Phryso, published in “The Friesian” July/August 2003 edition. [Equine Color: Chestnut] – A non-profit website which offers basic information on color genetics, including an example of a chestnut Friesian.