Say Hello! to White Prince, the newest white thoroughbred. He was born last Friday at Patchen Wilkes Farm in my hometown, Lexington, KY. White thoroughbreds are rare. Only 30 have been registered with the Jockey Club since 1963 when the first, a filly named White Beauty, was foaled at the same farm. His mom, Patchen Beauty, has given birth to three white foals, so far.
notoriously nice 
What chromosome determines coat color in horses. The report that you linked shows that a white foal could come from either a white sire or white mare. Is white dominant once the mutation is identified?
What am I thinking? I work for a major research university in the Genetics department…. OK, here is what I’ve found.
White is dominant or lethal.
w/w is fully pigmented.
W/w is white (no pigment)
W/W is lethal
Moral of the story, don’t mate two white horses.
That explains why more white horses are being registered every year.