• 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
  

Bay (ASIP)

• Last updated: 2026-01-14

  Common Names: Bay, Agouti 

  Scientific Name: agouti signaling protein (ASIP)

  Equine Chromosome: 22

  General Overview:

  The A gene (A for agouti) is responsible for controlling the distribution of black and red pigment. A horse must have at least one copy of dominant E for agouti to have any effect. [1] Horses with A/A or A/a and having at least one copy of dominant E, will have some amount of restriction of black pigment. This can range from nearly all black in the case of dark brown horses, to nearly no black pigment in the case of wild bay horses.
  
  Horses with the recessive a/a genotype, and having at least one copy of dominant E, will show no pheomelanin (red) pigment.
  
  Agouti status has no impact on e/e horses. 

  
  

  An older, now discredited idea, proposed that the wide variety of shades of bay was influenced by different polymorphs of the A gene. A^t was proposed for brown shades and A⁺ for wild bay shades. There is now strong evidence that the combined status of E and A play a large role in determining the final shade. [2] My own conjecture is that additional genetic and environmental factors almost certainly play a role in deciding the final shade as well.

• Citations:

  1. Rieder, S., Taourit, S., Mariat, D., Langlois, B., & Guérin, G. (2001). Mutations in the agouti (ASIP), the extension (MC1R), and the brown (TYRP1) loci and their association to coat color phenotypes in horses (Equus caballus). Mammalian Genome, 12(6), 450-455. doi: https://pubmed.ncbi.nlm.nih.gov/11353392/ 

   

  2. Shang S, Yu Y, Zhao Y, Dang W, Zhang J, Qin X, Irwin DM, Wang Q, Liu F, Wang Z, Zhang S, Wang Z. Synergy between MC1R and ASIP for coat color in horses (Equus caballus)1. J Anim Sci. 2019 Apr 3;97(4):1578-1585. doi: 10.1093/jas/skz071. PMID: 30785190; PMCID: PMC6447268. https://pubmed.ncbi.nlm.nih.gov/30785190/

• Examples:

  A range of bay expression from light to very dark