Seal Brown (horse) - Former Theories About The Genetics of Seal Brown - Extension-brown and Dominant Black

Extension-brown and Dominant Black

The allure of a pure black coat on a horse has struck horse breeders for centuries, resulting in all-black breeds like the Friesian horse. The breeding of pure black horses is attended by two problems: some black coats fade with exposure to light and sweat, and breeding two "black" horses together would sometimes produce non-black horses. In some cases, faded true black horses have lighter coats than the darkest near-black horses.

To account for this, W.E. Castle postulated that there was a third allele at the Extension locus: ED or "dominant black". Based on the existence of such conditions in other animals, Castle suggested that the dominant black gene (ED) would override the "points" pattern of dominant Agouti (A) and produce black or near-black horses, which could then go on to have bay offspring. The implication was that the seal brown coat color, which is often quite nearly black, could be produced by this allele.

Similarly, D.P. Sponenberg suggested an Extension-brown (EB) allele, dominant over the wildtype E. He described an allele responsible for black countershading, or sootiness, which would distinguish all shades of brown from all shades of bay.

Both theories were laid to rest after the characterization of the equine MC1R or Extension, which showed no such alleles. However, it remains likely that a genetic control for sootiness does exist.