Splashed White (SW1–SW8) In Horses: What Do These Variants Mean?

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Splashed White is a white spotting pattern associated with variable expression — often including a broad blaze, high white, belly spots, and frequently one or two blue eyes. Because there are multiple known Splashed White variants, it’s possible for a horse to carry more than one “SW” result at the same time. This article clarifies a common point of confusion: compound heterozygous results (like SW1/SW7) versus true homozygous results (like SW1/SW1).

One In A Million Dun: n/SW1, n/SW7 or SW1/SW7
One In A Million Dun: n/SW1, n/SW7 or SW1/SW7

Which genes are involved?

According to UC Davis VGL, the currently recognised Splashed White variants are found across two different genes:

  • PAX3 variants: SW2, SW4
  • MITF variants: SW1, SW3, SW5, SW6, SW7, SW8

Because these variants can exist in different genes (and multiple variants can exist within the same gene), a horse can test positive for more than one Splashed White variant.

Homozygous vs compound heterozygous (the core difference)

  1. True homozygous: two copies of the identical variant

    • A horse is homozygous when it carries two copies of the exact same variant, for example:

      • SW1/SW1 (MITF)
      • SW2/SW2 (PAX3)

      UC Davis notes that SW1/SW1 horses have been identified, supporting that SW1 is not homozygous lethal.

  2. Compound heterozygous: one copy each of two different variants

    • A horse is compound heterozygous when it has one copy of variant A and one copy of variant B (typically within the same gene), for example:

      • SW1/SW7 (both MITF variants)
      • SW2/SW4 (both PAX3 variants)

Important: Results written as SW1/SW7 can be misread as “homozygous” because of the slash format. Genetically, it is not homozygous — it is compound heterozygous (two different alleles). A true homozygous result would be SW1/SW1.

Why it matters for breeding

UC Davis explains that horses which are homozygous or compound heterozygous will pass a Splashed White allele to all offspring. That means a horse with SW1/SW7 bred to an N/N partner should produce foals that all inherit either SW1 or SW7 (typically in roughly 50/50 proportions).

Stacking variants: what research has observed

Peer-reviewed research has documented that “stacking” Splashed White variants can increase white expression, including reports of horses that are completely white in certain multi-variant combinations. Research has also described deafness in at least one completely white horse with multiple Splashed White alleles, highlighting why high-white outcomes warrant extra awareness.

Risks and cautions when breeding Splashed White horses

1) Deafness can occur (but is not guaranteed)

UC Davis states that some, but not all, Splashed White horses are deaf. If deafness is suspected, clinical evaluation is recommended. From a practical standpoint, breeders should be mindful that very high-white outcomes (including completely white foals) may be associated with higher risk.

2) Some genotypes may be embryonic lethal

UC Davis notes that homozygous forms of SW3, SW4, SW5, SW6, SW7, and SW8 have not been detected and may be embryonic lethal. They also note that certain compound combinations have not yet been identified in their tested population, and it is unknown whether this is due to rarity or non-viability.

3) Be cautious with PAX3-based Splash pairings

Research discussing PAX3-associated Splashed White variants reports that homozygous outcomes were not observed for at least one PAX3 Splash allele and cautions against breeding two carriers together due to concerns that homozygous embryos may not be viable (based on developmental biology evidence).

4) Industry-level caution about “double Splash” pairings

Some breeder-facing educational resources warn of reported congenital issues in certain “multiple Splash” pairings and encourage testing and avoiding heavy stacking until more is known. These reports are not a substitute for peer-reviewed causation, but they align with the broader theme: stacking white-pattern variants can increase risk and unpredictability.

How to write results clearly (to avoid confusion)

  • Homozygous: SW1/SW1 (homozygous SW1)
  • Compound heterozygous: SW1/SW7 (compound heterozygous; two different Splash variants)
  • Stacked across genes: SW1/N + SW2/N (MITF + PAX3)

If you’re publishing stallion profiles or sharing test panels on social, simply adding the words “homozygous” or “compound heterozygous” next to the result goes a long way in preventing misinterpretation.

Sources

  1. UC Davis Veterinary Genetics Laboratory (VGL) — Splashed White (SW1–SW8): https://vgl.ucdavis.edu/test/splashed-white
  2. Peer-reviewed research (NCBI/PMC) related to Splashed White genetics and outcomes: https://pmc.ncbi.nlm.nih.gov/articles/PMC7238438/
  3. WZ Equine — educational breeder-facing overview of Splash genetics: https://wzequine.com/splash-gene/