Mushroom: when red pigment looks sepia
The MFSD12 gene and its Mushroom version, Mu
Mushroom can turn a chestnut-based coat toward muted sepia tones. It is best documented in Shetland ponies and can resemble other dilutions.
Explanations and diagrams reviewed on 3 November 2026
Where the appearance is best documented
On chestnut, Mushroom reduces reddish tones, often leaving a muted sepia body with a lighter mane and tail. Shades vary and may resemble Cream or Silver. A color description alone cannot distinguish the causes.
The original study mainly examined Shetland ponies and also detected the variant in Miniature Horses. A similar-looking horse of another breed does not automatically have the same inherited variant.
Understanding Mu and N
MFSD12 is the scientific symbol for the gene involved, and Mu names its Mushroom version. The DNA change is written c.600dupC. An extra DNA letter changes how the instructions are read and is predicted to produce a shorter protein.
A horse inherits one allele, or version, from each parent. In this test, N means the Mushroom variant is absent on that copy. Mu/Mu means two copies and is associated with the dilution. N/Mu means one copy: the horse can pass it on without showing the usual dilution. This is recessive inheritance. N/N means no copies of the tested variant, while the gene itself is still present.
The limits on other base colors
Bay Mu/Mu ponies have also been observed, with less red in their body hair and black manes and tails retained. This appearance is less extensively documented than Mushroom on chestnut.
The study included very few observations of black-based horses. Those data cannot establish one Mushroom shade for every base color, breed or combination with other dilutions.
Example: two carriers
Two parents confirmed N/Mu each pass either N or Mu. Every conception has a 25% chance of Mu/Mu, 50% of N/Mu and 25% of N/N. Four births do not have to follow those proportions exactly.
Testing the base color and other plausible dilutions helps explain the final coat. A negative Mushroom result excludes this tested variant, not every genetic explanation for pale red hair.
An example: what could this foal inherit?
MFSD12
Both demonstration horses have the exact pair shown. These examples explain inheritance; they do not use the mare and stallion entered in the calculator.
MareN/Mu
StallionN/Mu
All possible pairs at each conception
- N/N25%
Two copies of N
- N/Mu50%
One N and one Mu
- Mu/Mu25%
Two copies of Mu
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 · Laboratory explanation
UC Davis VGL — Mushroom2 · Research paper
Tanaka et al. (2019) — MFSD12 explains Mushroom dilution in Shetland ponies