> ## 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. # Extension gene - URL: https://equinecolors.com/extension-gene/ - Published: 2025-11-05T13:28:56.000Z - Updated: 2026-08-10T09:50:55.000Z - Description: The extension gene controls the production of black and red pigment. It is one of the genes that determines the base color of horses. - Author: Hannelore Roodhooft - Tags: Genetics, Base colors, #Import 2026-08-12 09:42 The **extension** or **MCR1** (melanocortin 1 receptor)gene, also sometimes called **"black/ red factor"**, determines whether a horse will have a chestnut or a black/bay base coat color. ## Alleles The extension [gene](https://equinecolors.com/dictionary/#:~:text=in%20each%20pair.-,Gene,-A%20gene%20is) has three possible [alleles](https://equinecolors.com/dictionary/#:~:text=on%20a%20chromosome.-,Alleles,-Different%20versions%20of). The [dominant](https://equinecolors.com/dictionary/#:~:text=Dominant%20Allele%20%28written%20in%20capital%20letters%29) **E-allele** allows for the production of **black pigment** and leads to a **black** or **bay base color** depending on the **agouti** gene. The [recessive](https://equinecolors.com/dictionary/#:~:text=Recessive%20Allele%20%28written%20in%20lowercase%20letters%29) alleles **e and e**a make horses unable to produce black pigment. The result is **red pigment** only. e and ea have the same effect and can't be distinguished visually.. The ea ("alternate") allele is rare and has only been found in a few horse breeds. | **Extension** | **Color** | | ------------- | -------------------------------------- | | E/E | Black or bay base, depending on agouti | | E/e | Black or bay base, depending on agouti | | e/e | Chestnut base | | e/ea | Chestnut base | | ea/ea | Chestnut 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. ### Extension 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 **extension** gene codes for this **MC1R** receptor. [**Mutations**](https://equinecolors.com/dictionary/#:~:text=on%20a%20chromosome.-,Mutation,-A%20mutation%20is) in the gene can lead to a receptor that no longer works and can't be activatedat all, with **red pigment** as a result. There are two such mutations of the extension gene: the alleles **e** and **e**a. Horses that are **e/e, eᵃ/eᵃ** or **e/eᵃ** have **no functional receptors** and can produce only **red pigment**. The [wild-type](https://equinecolors.com/dictionary/#:~:text=for%20each%20gene.-,Wild%2Dtype%20allele,-The%20wild%2Dtype) **E-allele** leads to **normal receptors**.This makes it possible for horses that are **E/E** or **E/e** to produce **black pigment**. | **Alleles** | **Mutation** | **Effect** | | ----------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------- | | E | / | Black pigment is possible | | e | [Point mutation](https://equinecolors.com/genetics-2-alleles/#:~:text=%F0%9F%A7%AC-,Types%20of%20mutations%3A,-Point%20mutation%3A%20One)/ substitution (C => T)[missense mutation](https://equinecolors.com/genetics-2-alleles/#:~:text=%F0%9F%A7%AC-,Types%20of%20mutations%3A,-Silent%20mutation%3A%20No) | Red pigment | | ea | Point mutation/ substitution (G => A)missense mutation | Red pigment | ## Colors [ChestnutChestnut is one of the base coat colours in horses, next to black and bay. It is characterised by a fully red coat.![](https://equinecolors.com/content/images/icon/favicon-33.ico)Equine ColorsHannelore Roodhooft![](https://equinecolors.com/content/images/thumbnail/chestnuthorse-2.png)](https://equinecolors.com/chestnut/) [BlackBlack is one of the base colours in horses, next to chestnut and bay. It is characterised by a fully black coat.![](https://equinecolors.com/content/images/icon/favicon-34.ico)Equine ColorsHannelore Roodhooft![](https://equinecolors.com/content/images/thumbnail/blackhorse-2.png)](https://equinecolors.com/black/) [BayBay is one of the base coat colours in horses, next to chestnut and black. It is characterised by a red coat with black points.![](https://equinecolors.com/content/images/icon/favicon-32.ico)Equine ColorsHannelore Roodhooft![](https://equinecolors.com/content/images/thumbnail/bayhorse-2.png)](https://equinecolors.com/bay/) Read more: [Color genes](https://equinecolors.com/color-genes/) | [Agouti gene](https://equinecolors.com/agouti-gene/) [Back to base colors](https://equinecolors.com/base-colors/) ## Articles 1. Marklund, L., Moller, M. J., Sandberg, K., & Andersson, L.; **A missense mutation in the gene for melanocyte-stimulating hormone receptor (MCIR) is associated with the chestnut coat color in horses**, Mammalian Genome, 1996; [10.1007/s003359900264](http://doi.org/10.1007/s003359900264?ref=equinecolors.com) 2. Wagner, H-J., & Reissmann, M.; **New polymorphism detected in the horse MC1R gene**; Animal Genetics, 2000*;* [10.1046/j.1365-2052.2000.00655.x](http://doi.org/10.1046/j.1365-2052.2000.00655.x?ref=equinecolors.com) --- [![CTA Image](https://equinecolors.com/content/images/2025/10/Image-5.png)](https://www.horsereality.com/?ref=equinecolors.com) Learn while playing! Discover how genes shape colors and patterns in our realistic horse breeding game, Horse Reality [Play now ](https://www.horsereality.com/?ref=equinecolors.com)