Gray: the color at birth still matters

Follow the horse through time

A gray horse is born with a base such as chestnut, bay or black, sometimes already showing early white hairs. More white hairs appear over time; the pace and final appearance vary. The skin generally stays pigmented except at white markings or other areas of depigmentation.

An almost white adult gray differs from a horse born extensively white. Early photographs may help identify its original color, which still matters for inheritance.

Sources for this explanation [1] · [2] · [3]

Why the names include numbers

STX17 is the scientific symbol for the gene containing a short repeated DNA segment that helps control gene activity. N, also called G1, has one copy of that segment on the chromosome; G2 has two and G3 has three. The names describe alleles, or inherited versions. They do not count whole genes.

One G2 or G3 is enough for graying. The possible pairs are N/G2, N/G3, G2/G2, G2/G3 and G3/G3; N/N has neither graying version. Research associates G2 with slower graying and G3 with faster graying and greater susceptibility to melanoma. Rare G2/G2 and G2/G3 pairs remain poorly documented. None of these results predicts an exact timetable for an individual horse.

Sources for this explanation [1] · [2] · [3]

Example: two parents confirmed N/G2

Each N/G2 parent can pass N or G2. Every conception has a 25% chance of N/N, 50% of N/G2 and 25% of G2/G2. Overall, 75% inherit graying in this particular cross. A G2/G2 parent always passes G2.

A gray coat does not reveal whether the horse has one or two graying alleles. You therefore cannot apply this example’s percentages to every pair of gray parents.

Sources for this explanation [1] · [2] · [3]

What testing and health assessment can resolve

The Veterinary Genetics Laboratory at UC Davis, abbreviated VGL, offers a test that counts copies of the repeated DNA segment. A total of four does not distinguish N/G3 from G2/G2. Those pairs transmit graying differently, so an unresolved result must remain unresolved unless further evidence distinguishes them.

Gray is associated with susceptibility to melanoma, a tumor involving pigment cells. This association indicates a risk, rather than a diagnosis or certainty that melanoma will develop. A new or changing lump needs veterinary assessment, whatever the horse’s rate of graying.

Sources for this explanation [1] · [2] · [3]

Where the explanations come from

Laboratory resources explain which variants are tested and how to read their results. Research papers describe the evidence, discoveries and remaining uncertainties. Each article links its explanations to the relevant sources. Published titles stay in their original language so you can find the exact work.

1. 1 · Laboratory explanation

   UC Davis VGL — Gray Copy Number

2. 2 · Research paper

   Rosengren Pielberg et al. (2008) — STX17, graying and melanoma susceptibility

3. 3 · Research paper

   Rubin et al. (2024) — STX17 copy number, speed of greying and melanoma incidence