• 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
  

About This Guide

How This Guide is Being Developed

• Last updated: 2026-01-14

  
  

  This guide is in the process of being written, and I expect it to take at least all of 2026 to get the core parts of it finished. I expect it to always be evolving as more information is published. I apologize in advance if you run into unfinished sections. If you have an area you would like me to focus on finishing, or want me to expand on, please let me know.

  Here are my current progress goals:

  1. Complete the overview of all documented genes.
  2. Complete the overview of all hypothetical and fantasy genes.
  3. Complete the overview of non standard expression
  4. Complete the Glossary of Colors
  5. Complete the overview of genetic disorders
  6. Write up in-depth information about the functioning of documented gene
  
  

  I am also looking to expand the Glossary of Terms as I find new ones that need to be added. The in-depth knowledge write ups will probably also be done throughout the process as the mood strikes me. These will be the most complex and intense parts of the guide so spacing them out will help prevent burn out. 

Philosophy of this guide

• Last updated: 2026-01-24

  
  

  This guide is primarily written to help the members of HorseGeneticsGame.com with breeding their horses in game. I hope that the information presented here will be usable both to those who play HGG and those that do not. With that in mind I will be writing with a focus on real life science instead of how things are done in game. When information is specific only to the game or does not apply to the game it will be labeled as such. 

  
  

  This guide seeks to take a scientific approach rather than a dogmatic one. That means the information given is never taken as an absolute truth but instead it should be viewed as the most accurate information as it is understood at the time of writing. The information presented will always include citations so you can read the information yourself, and will label conjecture when it is presented.  

  
  

  This guide seeks to understand equine genetics in the larger context of all mammalian genetics. Almost all genes are shared between mammal species and what is learned from one species helps inform the bigger picture of what is going on with another. 

  
  

  This is a living document, that means it will be updated as scientific knowledge develops over time or as clarification is needed. The information presented in this guide will inevitably have errors, both because no one person can know everything but also because genetics is a science that is ever developing. If you have corrections or new information to share I would love to see it. I just ask that you provide a peer reviewed source and share it in a kind manner. There is no reason why we can not all seek knowledge together. 

• Terminology

  This guide aims to help the reader stop thinking of color genetics like cars slotting into garage bays, and to instead think about it like typos in a book. You can keep accumulating typos but eventually whole passages of the book may no longer make sense. With this in mind this guide will often use the term mutation as a reminder that there is an error in this version of that gene.

  
  

  It is more technically correct to say allele, as that term means the whole of the gene, inclusive of any mutations we are discussing. Many geneticists will also use the terms polymorph or haplotype in a similar way to allele. The specific nuances between each of these terms isn't vital for the lay learner to understand. Just know that you will see the terms mutation and allele both in this guide and in both cases I am talking about a specific variant of that gene.

  
  

Using this Guide

• Last updated: 2026-01-14

  This guide is divided into Documented Genes, Hypothetical Genes, Fantasy Genes, Genetic Disorders and Nonstandard Expression. Documented genes have been identified by geneticists and their location published in academic papers. Often testing is available for these genes. 

  
  

  Hypothetical genes range from well supported genes that almost certainly exist in horses but have not been documented yet, to unsupported genes which almost certainly do not exist in horses but could theoretically be possible using genetic engineering. Fantasy genes are scientifically impossible genes found in HorseGeneticsGame.com and are not intended to reflect any real life genes.

  
  

  Genetic Disorders are found in real life horses and are important for all horses lovers to be educated on but are not found in HorseGeneticsGame.com. Nonstandard expression includes somatic mutations, brindle and other oddities.

  
  

  If you are a HGG member all of the categories except Genetic Disorders are useful for you to be acquainted with. If you are looking to learn about real horses the Documented and Genetic Disorders categories are the most important to learn about, with the Well Supported Hypothetical Genes also being very useful to learn.

Thinking Scientifically

• Last updated: 2026-01-14

  What makes thinking dogmatic?

  An idea is dogmatic when it is applied with rigid certainty, often because an authority figure, or peers said it was the truth. When someone is thinking dogmatically they don't apply a spirit of critical enquiry to information they have learned. They don't question the biases or larger context of the information. That information might still be true. The way of thinking about something does not make it true or false. However, rigid dogmatic thinking is the opposite of scientific thinking.

  
  

  What makes thinking scientific?

  Scientific thinking weighs the data presented against previously gathered evidence, applies careful observation, and most importantly revises its conclusions as new data is presented. We should all strive to approach genetics as a science that is ever growing and evolving in knowledge and never as a universal truth to be applied. Be wary of those who insist something is true without providing evidence and sources.

About the Author

• Last updated: 2026-01-14

  This guide is written by Ellie Akers, also known as Ammit, the creator of HorseGeneticsGame.com. Ellie is a well educated lay person but not a geneticist. Ellie started her first genetic experiments in 4th grade by breeding a population of purple striped starflowers from all white original stock, in her backyard. Once she learned about horse genetics young Ellie was hard at work organizing her toy horses into genetically plausible families.  Ellie has been studying horse genetics as a hobby for over 30 years and has studied inheritance first hand while breeding horses, goats, ducks, chickens, and of course plants.