It’s not infrequent as a sighting of Halley’s Comet, visible on Earth only once every 75 years, but what happened Monday at Mahoning Valley Race Course is still a racing rarity. Wear the Mask, whose color is officially listed as white by the Jockey Club, recorded his second lifetime victory in a first-level allowance in Monday’s fourth race. Ridden by Megan Fadlovich for trainer Ivan Vazquez, the 4-year-old Wear the Mask ($19.40) covered a mile and 70 yards on a sloppy, sealed track in 1:46.42, winning wire to wire by two lengths. What makes a white horse different from a light gray or roan? According to the Jockey Club, “The entire coat, including mane, tail, and legs is predominately white.” A white coat is not considered albino, but rather a genetic mutation carried by a single dominant gene. The list of successful white Thoroughbred racehorses has been minimal. In fact, no registered white Thoroughbred has ever won a stakes race in North America, although a Japanese-bred named Yukichan won three stakes from 2008-2010. Less than 1 percent of all registered Thoroughbreds are listed as white.