UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory

Mushroom Dilution of Shetland Ponies

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Discovered at the Veterinary Genetics Laboratory

Mushroom is a dilute coat color found in Shetland Ponies that is characterized by a distinctive sepia-toned body hair color, often accompanied by a flaxen mane and tail. The mushroom phenotype shows wide variation in shade and has similarities to cream and silver dilutions, although it is genetically distinct from both. Mushroom dilutes red pigment (pheomelanin) and thus the dilute phenotype is most obvious on a chestnut base color (Figure 1).

Photo Credits:Christine Mirjam Sørli and Maria Tammi

Figure 1. Mushroom dilution of the coat, mane and tail. The pony on the left is chestnut and the pony on the right is mushroom. Two copies of the mushroom variant in a chestnut pony dilute the red pigment of the coat, mane and tail.

Researchers at the Veterinary Genetics Laboratory and the University of Bern investigated the molecular basis of mushroom color dilution and identified a single nucleotide change in a pigmentation gene that explains this phenotype. This variant has also been identified in Miniature Horses. Pedigree analysis suggests an autosomal recessive mode of inheritance for mushroom dilution. This means that the phenotype occurs in both males and females but only chestnut ponies with two copies of the mushroom variant have the characteristic mushroom dilute phenotype.

Recent work at the Veterinary Genetics Laboratory suggests that bay horses homozygous for the mushroom variant may also have altered pigmentation in the coat. More information wil be available when the manuscript is published.

The VGL offers a DNA test for mushroom color dilution. Test results help owners make breeding decisions. If the mushroom phenotype is desired, it is advisable to breed mushroom ponies to each other (e/e, Mu/Mu genotypes). Mating of two chestnut mushroom carriers (e/e, Mu/N genotypes) will result in a 25% chance of producing a mushroom pony. Testing for Mushroom and other coat color genes at the VGL (Agouti, Red Factor, Silver, Dun, Cream) will help owners to distinguish mushroom ponies from similar phenotypes caused by other genes.

Testing recommended for: Shetland Pony, Miniature Horse

Results reported as:


No copies of mushroom variant detected.


1 copy of mushroom variant detected. The horse is a carrier and not dilute. If chestnut, it can produce mushroom foals when bred to the appropriate mate.


2 copies of mushroom variant detected. Chestnut horses with this genotype have mushroom coat color.


Frederiksen AG (2011). Genetic testing of silver Shetland Ponies for allelic variants of MC1R, ASIP and PMEL17 (Master’s Thesis). University of Copenhagen, Copenhagen, Denmark.

Sponenberg P & Bellone RR (2017). Equine Color Genetics, 4th Edition. Hoboken, NJ: Wiley-Blackwell.

Tanaka J, Mack M, Leeb T, Flury C, Backmann I, Penedo MCT & Bellone RR (2019). Genome Wide Association Study Identifies Locus for the Mushroom Coat Color Dilution in Shetland Ponies. XXVII Plant and Animal Genome Conference, San Diego. Abstract number W359.

Veterinary Genetics Laboratory, Tel 530-752-2211, Email VGL