How horse coat color can be linked to health

Why one variant can affect both coat color and another function

Hair and skin color depend partly on pigment-producing cells called melanocytes. Some genes involved in their function or development also have roles in the eyes, inner ear or other tissues. A variant, a difference in the DNA sequence, can therefore have several effects. This is called pleiotropy.

A disease directly linked to a genetic combination differs from a predisposition that increases risk without making disease certain, or sensitivity associated with unpigmented skin. These situations need different interpretations. The number of variant copies can matter, but one copy does not have the same meaning for every variant. In the results shown here, N denotes a version without the tested variant; the gene is still present.

Sources [4] · [6] · [10]

Frame overo: one copy can change the coat, while two cause LWO

The frame overo variant in EDNRB, written O in this example, can produce white patches with just one copy. Its expression varies, and some horses show very little white. With two copies, the foal has lethal white overo syndrome, or LWO. The defect affects the development of nerve cells in the intestine, preventing it from moving its contents normally.

The foal does not die because its coat is white. Other mechanisms produce very pale or white foals without this syndrome, including the double Cream dilution in cremellos. Avoiding two copies of frame overo requires comparing both parents’ test results. An apparently solid coat cannot rule out this variant. A newborn foal with colic needs prompt veterinary assessment whatever its color.

Sources [1] · [2]

Leopard complex: night blindness and uveitis risk are different conditions

The leopard complex variant, written LP, affects TRPM1 and contributes to appaloosa-type patterns. Two copies, LP/LP, are associated with congenital stationary night blindness: the horse has difficulty seeing in dim light from birth, without progressive worsening due to this condition. One copy, N/LP, does not cause this form of night blindness. The amount of white and the number of spots cannot replace a DNA test.

Leopard complex is also associated with an increased risk of recurrent uveitis, inflammation inside the eye, particularly documented in LP/LP horses. This predisposition does not mean that all will develop uveitis. Night blindness and uveitis are separate conditions. Other forms of night blindness occur without LP and are not covered by this test.

Sources [3] · [4]

Silver: eye abnormalities can occur even when the coat dilution is hidden

The Silver variant in PMEL, written Z, lightens black pigment and is associated with multiple congenital ocular anomalies, or MCOA. One copy can already be accompanied by abnormalities, sometimes subtle cysts. Two copies are generally associated with more extensive abnormalities. Their occurrence and effect on vision vary; the DNA test does not measure how well a horse sees.

In a chestnut horse, Silver’s coat effect can remain hidden because there is no black pigment in the hair for it to lighten. This does not remove the variant or its possible effects on the eyes. A chestnut can therefore pass on Silver and may need an eye examination. A light mane alone also does not establish Silver, since it can have other causes.

Sources [5] · [6]

Gray: progressive graying is linked to a predisposition to melanoma

A gray horse is born with a pigmented coat that progressively develops white hairs; the skin usually remains dark. Variants in the STX17 region are linked to this graying and to susceptibility to melanoma, tumors of pigment cells. The association does not mean a gray horse already has a tumor, or that a lump in a horse of another color can be ignored.

Research distinguishes G2, associated with slower graying and a low incidence of melanoma in the populations studied, from G3, associated with faster graying and greater risk. These findings are not universal individual probabilities. The test counts repeats of a short DNA sequence; some results cannot distinguish N/G3 from G2/G2. A DNA test does not diagnose a tumor; a new or changing lump needs examination.

Sources [7] · [8]

Some splashed white patterns are associated with deafness

Splashed white patterns can combine a broad blaze, extensive white leg markings and blue eyes. Different variants in MITF or PAX3 can cause these patterns. Some affected horses are deaf and others hear normally. Pigment cells also contribute to normal inner-ear function, which helps explain the connection.

Neither the amount of white nor eye color can establish whether a horse hears normally. The consequences depend on the variant and sometimes on combinations with other variants. A white-pattern test cannot replace a veterinary hearing assessment. A risk associated with one variant must not be extended to all white patterns.

Sources [9] · [10]

Pink skin and sunlight: skin sensitivity is not proof of inherited disease

Unpigmented skin, often pink under white markings, is more sensitive to ultraviolet light, especially where hair coverage is sparse, such as the muzzle. This is a sensitivity of the exposed skin, rather than evidence of a genetic disease. An almost white gray horse can still have dark skin, so hair color alone does not describe this sensitivity.

Access to shade and suitable UV protection can reduce exposure of sensitive areas. Redness, crusting or substantial skin lesions should not automatically be attributed to coat color: photosensitization, for example involving certain plants or a health problem, can also contribute. A veterinarian investigates the cause and tailors care instead of diagnosing from color alone.

Sources [11]

Using coat information to choose tests and monitoring

The coat and family history can help guide which questions to ask. They do not provide all genetic results. Ask which variant the laboratory tests for, how it is written and what zero, one or two copies mean in that particular case.

Before breeding, compare both parents’ results and the variant’s particular effects. Avoiding two copies of frame overo prevents the intestinal LWO syndrome caused by that variant. Avoiding two copies of Silver does not rule out the eye abnormalities possible with one copy. The coat calculator estimates appearance within its model; it does not assess the future foal’s overall health.

Sources [2] · [5] · [7] · [11]

Questions about coat colors and health

Sources on horse coat colors and health

These references combine research on variants, UC Davis clinical resources and IFCE guidance. The links described apply to the mechanisms studied; they do not rank every coat color by an overall disease risk.

1. 1.UC Davis VGL — Frame overo testing and LWO interpretation
2. 2.UC Davis — Lethal white overo syndrome: mechanism and prevention
3. 3.UC Davis VGL — Leopard complex, night blindness and uveitis
4. 4.Bellone et al. (2013) — TRPM1, leopard complex and night blindness
5. 5.UC Davis VGL — Silver and multiple congenital ocular anomalies
6. 6.Andersson et al. (2013) — PMEL effects on Silver coat color and eyes
7. 7.UC Davis VGL — Gray variants, copy number and melanoma
8. 8.UC Davis — Melanoma in horses
9. 9.UC Davis VGL — Splashed white variants and hearing
10. 10.Hauswirth et al. (2012) — MITF, PAX3 and splashed white patterns
11. 11.IFCE — Unpigmented skin, ultraviolet light and photosensitization