Hmmm, I was perusing another thread and came across this information that I thought some of you may be interested in.
According to UC Davis (who performs many tests for color and genes):
Roan Zygosity Test
Roan is a white patterning coat color trait of intermixed white and colored hairs in the body while the head, lower legs, mane and tail remain colored. Roan horses are born with the pattern, though it may not be obvious until the foal coat is shed. The white and colored hairs are evenly mixed in horses that inherit the classic Roan gene, which can differentiate this from several mimic patterns called roaning. Roaning patterns tend to be uneven in the distribution of white hairs and the inheritance of roaning has not been defined. The Roan gene is found in a variety of breeds such as Quarter Horse, Paints, Peruvian Paso, Paso Fino, Welsh Pony, Miniature and Belgian, but not in Thoroughbreds or Arabians.