> ## Content Index > Fetch the complete content index at: https://equinecolors.com/llms.txt > Use this file to discover other available public pages before exploring further. # Agouti gene - URL: https://equinecolors.com/agouti-gene/ - Published: 2025-11-06T10:26:12.000Z - Updated: 2026-08-10T09:51:21.000Z - Description: The agouti gene controls the distribution of black on the coat. It is one of the genes that determines the base color of horses. - Author: Hannelore Roodhooft - Tags: Base colors, Genetics, #Import 2026-08-12 09:42 The Agouti gene or ASIP (Agouti Signalling Protein) determines whether horses will have a bay or black base coat color. ## Alleles The agouti [gene](https://equinecolors.com/dictionary/#:~:text=in%20each%20pair.-,Gene,-A%20gene%20is) has two possible [alleles](https://equinecolors.com/dictionary/#:~:text=on%20a%20chromosome.-,Alleles,-Different%20versions%20of). Agouti only affects horses with **E/E E/e** for [extension](https://equinecolors.com/extension-gene/); since chestnut-based horses (e/e) aren't able to produce any black pigment, agouti has no impact on their color\*. The [dominant](https://equinecolors.com/dictionary/#:~:text=Dominant%20Allele%20%28written%20in%20capital%20letters%29) **A-allele** restricts **black** pigment (if present) to the **points** of the horse, resulting in a **bay base color.** The [recessive](https://equinecolors.com/dictionary/#:~:text=Recessive%20Allele%20%28written%20in%20lowercase%20letters%29) **a-allele** allows black pigment (if present) to be uniformly distributed over the coat, which results in a **black base coat**. | **Extension** | **Agouti** | **Color** | | ------------- | ---------- | ---------- | | E/E | **A/A** | Bay base | | E/e | **A/A** | Bay base | | E/E | **A/a** | Bay base | | E/e | **A/a** | Bay base | | E/E | **a/a** | Black base | | E/e | **a/a** | Black base | ## Mechanics Pigment cells, called **melanocytes**, produce the pigments in the hair and skin of horses. The production of pigments can be influenced at different stages, changing the **type (black or red)** or the **amount (dilutions/ white patterns)** of pigment that is made. ![CTA Image](https://equinecolors.com/content/images/2025/12/Equinecolors_Icon_DNA1-10.png) The production of black and red pigment involves several different steps. Some of these steps are shared between the two, and some are unique to each pigment type. This is why certain genes will affect all base colors, while others only affect either red or black pigment: they affect different steps. ### Agouti Melanocytes can produce two types of pigment: **eumelanin (black)** and **pheomelanin (red)**. Which pigment is made depends on a receptor on their surface, called **MC1R**. This receptor can be activated by a protein known as MSH. - When the receptor is **activated** by MSH, **black** pigmentis formed. - When there is **no activation** by MSH, cells produce **red** pigment. The **agouti** gene codes for a protein called **ASIP**. This protein **blocks** the **receptors** so that MSH can't get to them. As a result, the receptors can't be activated, and the affected areas will produce **red pigment**. The **a-allele** is a [mutation](https://equinecolors.com/dictionary/#:~:text=on%20a%20chromosome.-,Mutation,-A%20mutation%20is) that makes this ASIP protein non-functional, so it can no longer block the receptors. Horses that are **a/a** can produce black pigment all over the body (if E/E or E/e). The [wild-type](https://equinecolors.com/dictionary/#:~:text=for%20each%20gene.-,Wild%2Dtype%20allele,-The%20wild%2Dtype) **A-allele** produces a normal ASIP protein. In horses that are **A/A** or **A/a,** **black** pigment will be **restricted** to the **points** of the horse as a result. | **Alleles** | **Mutation** | **Effect** | | ----------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------- | | A | / | Black pigment is restricted to the points | | a | 11 bp[ deletion ](https://equinecolors.com/genetics-2-alleles/#:~:text=%F0%9F%A7%AC-,Types%20of%20mutations%3A,-Point%20mutation%3A%20One)[Frameshift mutation](https://equinecolors.com/genetics-2-alleles/#:~:text=%F0%9F%A7%AC-,Types%20of%20mutations%3A,-Silent%20mutation%3A%20No) | Black pigment over the entire body | ## Colors [BlackBlack is one of the base colours in horses, alongside chestnut and bay. It is defined by a fully black coat.![](https://equinecolors.com/content/images/icon/favicon-39.ico)Equine ColorsHannelore Roodhooft![](https://equinecolors.com/content/images/thumbnail/blackhorse-4.png)](https://equinecolors.com/black/) [BayBay is one of the base coat colours in horses, alongside chestnut and black. It is defined by a red coat with black points in a wide range of shades.![](https://equinecolors.com/content/images/icon/favicon-42.ico)Equine ColorsHannelore Roodhooft![](https://equinecolors.com/content/images/thumbnail/bayhorse-5.png)](https://equinecolors.com/bay/) Read more: [Color genes](https://equinecolors.com/color-genes/) | [Extension gene](https://equinecolors.com/extension-gene/) [Back to base colors](https://equinecolors.com/base-colors/) ## ## Articles 1. Rieder, S., Taourit, S., Mariat, D., Langlois, B., Guerin, G.; ***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 (2001) Doi: [10.1007/s003350020017](https://link.springer.com/article/10.1007/s003350020017?ref=equinecolors.com) \*e refers to both e and ea alleles for simplicity, as they both have the same effect