Genetics
Varnish roan is thought to occur due a single, simple dominant gene on equine chromosome 1 (ECA1). It also appears that specific white patterning genes produce the assorted blanket, leopard, and snowflake coat patterns. Without these white patterning factors, horses with one or two copies of the dominant Lp gene are "varnish roans." Varnish roans can be considered to have many, many very small leopard spots that reflect the mottling of the underlying skin.
A horse may be varnish roan as well as true roan and gray, but such combinations are extremely uncommon. The breeding of grays is discouraged by the Appaloosa Horse Club and not allowed by Knabstrupper breed registry.
Read more about this topic: Varnish Roan