G R E Y
Grey (or gray) is the COR018 gene on Chromosome 25 that, when a dominant allele is present, activates a cellular disease that affects any underlying color. It causes cells to overproduce pigment, which can make a coat become darker (called hyperpigmentation). The cells then "burn out" and produce less and less pigment, which results in the hairs becoming more and more grey over time until the coat is completely white in color. Some horses will acquire colored ticking called fleabites. This is when some cells reactivate or retain their ability to produce pigment. The color of fleabites does not necessarily correlate with what color the horse was before greying.
Sometimes the coat of a foal that will grey is a more "adult" color than non-grey counterparts. They may also have grey goggles around they eyes that can indicate grey. The process typically begins in the face and tail, spreading over the body. Hyperpigmentation can cause pseudo-primitive markings, which may confuse some into thinking their horse is dun. Given enough time, all horses with a dominant grey allele will become completely white in color. Every horse will grey out at their own rate, some are very fast and others may take many, many years.
Sometimes the coat of a foal that will grey is a more "adult" color than non-grey counterparts. They may also have grey goggles around they eyes that can indicate grey. The process typically begins in the face and tail, spreading over the body. Hyperpigmentation can cause pseudo-primitive markings, which may confuse some into thinking their horse is dun. Given enough time, all horses with a dominant grey allele will become completely white in color. Every horse will grey out at their own rate, some are very fast and others may take many, many years.
GG or Gg