Every gray horse has at least one gray gene that depigments the base color over time. So every hair is affected though the rate of graying and the uniformity are different between horses. Some will be steel gray all of their lives, while others will become solid white. However, all gray horses keep their original skin color of chestnut or bay/black. Having a base color in a gray horse is the difference between a horse that has grayed into a solid white horse and a true white horse that lacks pigment in its skin.
A horse with either a GG or a Gg gene combination will be gray, while horses that never gray are gg. Breeding a GG to another GG or a Gg will always yield a gray horse. Breeding a Gg to a Gg will give 75% gray (one GG and two Gg) and 25% non-gray (gg). Breeding a GG to a gg will yield 100% gray foals. Breeding a Gg to a gg will produce 50% grays (two Gg) and 50% non-gray (two gg’s).
Gray horses can over-express two genes (STX17 and NR4A3) which will cause benign melanoma tumors. Melanoma of the skin is the horses’ most common skin tumor and is limited to black skin and almost always to gray horses. It is unknown why these two genes overexpress in gray horses, but a high percentage of gray horses will develop these hard, black lumps in the skin. These tumors can become metastatic in rare cases, spreading throughout the body and becoming life-threatening.
The term “flea-bitten” is where some base colors remain as small speckles covering the whole body. These usually disappear over time.
Dapples are common in young horses that are starting to turn gray. These are colored patches with a light gray lace forming over the body. Dapples can be in any colored horse but are striking in the gray. They are a sign of good health as horses with a dull hair coats will never dapple.
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