Splashed white

What it looks like

A strongly marked horse may look as though its legs and lower body were dipped in white. It may have clear borders to the white areas, a broad white facial marking and blue eyes. Other horses show only modest facial and leg markings. Neither blue eyes nor a broad white facial marking proves splashed white on its own, and a carrier need not look like the typical example.

Sources for this explanation [4]

What happens biologically

MITF and PAX3 are two genes involved in directing the development of pigment cells. A variant is a particular version of the DNA sequence; different variants in either gene can produce similar splash markings. SW means splashed white, and the number identifies a specific variant. It is not a scale of increasing white.

The UC Davis SW1–SW8 panel tests eight named variants. SW2 and SW4 concern PAX3; SW1, SW3, SW5, SW6, SW7 and SW8 concern MITF. Because two different genes are involved, a horse can carry a splash variant in each gene. A panel is a selected group of tests, not a search for every possible cause of white markings.

SW9 in MITF and SW10 in PAX3 were also described in 2023 in a Pura Raza Española family. A panel labeled SW1–SW8 therefore does not cover every published variant. Findings in one family do not establish how common these variants are across all horses or every consequence of inheriting two identical copies.

Sources for this explanation [5] · [3]

Inheritance and DNA testing

For several splash variants, one copy is enough to affect the coat, showing dominant expression. A horse with two identical copies, one from each parent, is homozygous for that variant. Horses with two copies have been documented for some variants. For others, this combination has not been observed or raises concern about whether the embryo can survive. The result must name the specific variant: there is no rule that all two-copy splash horses are healthy, or that all are lethal.

For example, a horse with one confirmed SW1 copy has a one-in-two chance of passing it on at each conception. The other parent may also contribute SW1 or a different white variant, so that chance alone does not predict how white the foal will be. Combinations within one gene and between MITF and PAX3 need separate interpretation.

Sources for this explanation [4] · [6]

Confusions, limits and health

Some splashed-white horses are deaf, but many hear normally. Pigment cells also support inner-ear function, and their absence can affect hearing. Coat and eye color cannot determine an individual horse's hearing. A veterinarian may recommend BAER, a test that records electrical responses in the auditory pathway after sound stimulation. A negative DNA panel excludes only the variants included, since other causes of similar markings exist.

Sources for this explanation [5] · [4] · [1] · [2]

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 · Research paper

   Magdesian et al. (2009) — Evaluation of deafness in American Paint Horses by phenotype, brainstem auditory-evoked responses, and endothelin receptor B genotype

2. 2 · Research paper

   Aleman et al. (2014) — Brainstem auditory evoked responses in an equine patient population: Part I — Adult horses

3. 3 · Research paper

   McFadden et al. (2023) — Two Novel Variants in MITF and PAX3 Associated With Splashed White Phenotypes in Horses

4. 4 · Laboratory explanation

   UC Davis VGL — Splashed White Mutations SW1–SW8

5. 5 · Research paper

   Hauswirth et al. (2012) — Mutations in MITF and PAX3 cause splashed white and other white spotting phenotypes in horses

6. 6 · Research paper

   Magdesian et al. (2020) — De novo MITF deletion explains a novel splashed white phenotype in an American Paint Horse