Recently I was reading about equine gene testing on the University of California-Davis website when I came across a test for the “mushroom gene.” I’m familiar with many color modifier genes in horses — cream, dun, champagne, and even rarer ones like silver dapple and pearl. But “mushroom” was one I hadn’t heard of, so I had to learn more about it.

Bay mushroom Shetland pony

At first glance I would describe this pony as a sooty buckskin, but he tested homozygous for mushroom on bay with no cream gene. Mushroom-colored horses often have dark “countershading” on the head, neck, shoulders, and back. Photo by Mira Ruotsalainen

The mushroom gene was identified around 2014 as a red modifier found mainly in Shetland ponies. One reason mushroom wasn’t identified earlier is that it sometimes resembles silver dapple, but they’re distinct genes. Here are the differences:

Mushroom is probably recessive, while silver dapple is dominant.

  • The silver dapple dilution gene is dominant, so when a horse inherits one copy of the gene, it expresses it
  • The mushroom gene is believed to be recessive, so it requires a horse to inherit the mushroom gene from both parents in order to express it
  • Because the mushroom gene is recessive, a horse can carry the gene and pass it to descendants without expressing it

Palomino mushroom ponies

These two ponies have both a single cream gene and two mushroom genes applied to chestnut. Interestingly, they don’t express a light dilute color you might expect from palomino. They both have dark countershading on their heads from the mushroom gene. Left photo by David Hodge, right photo from the NIH study

Every mushroom-colored horse will pass a copy of the gene to its offspring.

  • A horse displaying the mushroom color is homozygous for the gene (meaning it has two copies), so every foal will inherit a mushroom gene from that parent
  • Breeding two mushroom-colored horses will always result in mushroom-colored foals

Dark bay mushroom pony

A dark bay Shetland pony with two mushroom genes. Photo by Katrin Lach

The mushroom gene only affects red pigment, while the silver dapple gene only affects black and brown pigment.

  • Mushroom typically modifies chestnut horses to a mousy body color with light manes and tails
  • Mushroom modifies bay body colors to a mousy color similar to the chestnuts, but mushroom doesn’t affect black manes and tails
  • Silver dapple lightens black and brown colors but doesn’t affect red
  • A black horse with the silver dapple gene will appear a chocolate color with lighter mane and tail, and a bay horse with the silver dapple gene will often look like a chestnut with flaxen mane and tail, but with dark legs

Buckskin mushroom Shetland pony

Mushroom on buckskin. While mushroom applied to palomino doesn’t seem to lighten coat color much, mushroom applied to buckskin can lighten the coat quite a bit. Photo by Claudia Rahlmeier

What coat colors aren’t affected by these genes?

  • In theory, you wouldn’t see the mushroom gene affect black horses, even if they are homozygous for the gene, because they don’t have a red base color
  • A chestnut horse could have the silver dapple gene, but you wouldn’t know it without genetic testing because silver dapple has no effect on red/chestnut

DNA testing has shown that mushroom can occur in Shetlands, Miniature Horses, and Icelandic Horses. In addition, the gene was found “in a heterozygous state” in Quarter Horses and Thoroughbreds, meaning the tested horses carried one mushroom gene. Eventually we may see homozygous mushroom-colored horses in those and their related breeds, like Paint Horses and Appaloosas.

Chestnut mushroom pony

A chestnut mushroom Shetland pony. Notice the dark “countershading” on the head. Photo by Maria Tammi

Researchers are still studying the newly discovered mushroom gene to learn about any adverse health issues associated with the gene, as well as the interaction of mushroom with other color genes.

If you think your pony might be a mushroom gene carrier, UC Davis has a low-cost test that will give you a definitive answer. Read more in the research study, Frameshift Variant in MFSD12 Explains the Mushroom Coat Color Dilution in Shetland Ponies.

Read more about the silver dapple gene and view a gallery of breeds that carry the color gene.