• About This Guide
  
• How This Guide is Being Developed
  
• Philosophy of this guide
  
• Using this Guide
  
• Thinking Scientifically
  
• About the Author
  
• Documented Genes
  
• What is a documented gene?
  
• Black/Red (MC1R)
  
• Bay (ASIP)
  
• Cream/Pearl (SLC45A2)
  
• Dun (TBX3)
  
• Gray (STX17)
  
• Silver (PMEL17)
  
• Champagne (SLC36A1)
  
• Roan/Tobiano/Sabino/White Spotting (KIT)
  
• Splash White (MITF/PAX3)
  
• Leopard Complex (TRPM1/ECA3P)
  
• Frame (EDNRB)
  
• Tiger Eye (SLC24A5)
  
• Height Regulation (HMGA2/LCORL)
  
• Mushroom (MFSD12)
  
• Hypothetical Genes
  
• What is a hypothetical gene?
  
• Borrowed Genes
  
• Glossaries
  
• Glossary of Colors
  
• Glossary of Terms
  

Dun (TBX3)

• Last updated: 2026-01-14

  Common Names: Dun

  Scientific Name: T-box transcription factor 3 (TBX3)

  Equine Chromosome: 8

  General Overview:

  The TBX3 gene has three known forms in horses. Dun (D), non-dun 1 (nd1) and non-dun 2 (nd2). Dun is believed to be the more ancient form of the gene. Non-dun 1 is a mutation that results in less diluted pigment and less primitive markings than dun. Non-dun 2 is an additional mutation that happened on the non-dun 1 variant to produce a version with no dilution and primitive markings at all. [1]

  
  

  Dun is the most dominant form. Dun turns black horses grullo, bays into bay duns and chestnuts into red duns. Dun dilutes the body color of the horse, to a more buff, tan, mousy or golden tone than the original color. Dun horses normally have darker faces, legs, manes, and tails than their body colors. They will have a distinctly darker dorsal stripe running the full length of their spine and darker bars on their ears. Many duns have additional "primitive markings” like forehead cobwebs, shoulder capes or stripes, and leg barring.

  
  

  Non-dun 1 is the next most dominant. It results in very minimal body dilution that may not be discernible from normal coloration by eye. Non-dun 1 horses normally have the suggestion of a dorsal stripe which is not as clear as true dun dorsal stripes.

  
  

  Non-dun 2 is the least dominant. This is the typical coloration and causes no dilution or primitive markings.

  
  

• Citations:

  1. Imsland, Freyja & McGowan, Kelly & Rubin, Carl-Johan & Henegar, Corneliu & Sundström, Elisabeth & Berglund, Jonas & Schwochow, Doreen & Gustafson, Ulla & Imsland, Páll & Lindblad-Toh, Kerstin & Lindgren, Gabriella & Mikko, Sofia & Millon, Lee & Wade, Claire & Schubert, Mikkel & Orlando, Ludovic & Penedo, Cecilia & Barsh, Gregory & Andersson, Leif. (2015). Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses. Nature Genetics. 48. 10.1038/ng.3475. https://pubmed.ncbi.nlm.nih.gov/26691985/

• Examples:

  Grullo E/? a/a D/?, Bay Dun E/? A/? D/?, Red Dun e/e D/?

  

  
  

  Black with nd1 E/? a/a nd1/?, Bay with nd1 E/? A/? nd1/?, Chestnut with nd1 e/e nd1/?