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# TBX3 gene
- URL: https://equinecolors.com/tbx3-gene/
- Published: 2025-11-26T11:04:27.000Z
- Updated: 2026-08-10T10:42:44.000Z
- Description: The TBX3 gene is responsible for the dilution dun and non-dun1.
- Author: Hannelore Roodhooft
- Tags: Genetics, Dilutions, #Import 2026-08-12 09:42

## Alleles

The TBX3 [gene](https://equinecolors.com/dictionary/#:~:text=in%20each%20pair.-,Gene,-A%20gene%20is) has three currently known [alleles](https://equinecolors.com/dictionary/#:~:text=the%20DNA%20sequence-,Alleles,-Different%20versions%20of) that influence coat color.

The [dominant](https://equinecolors.com/dictionary/#:~:text=Dominant%20Allele%20%28written%20in%20capital%20letters%29) **D-allele (dun)** dilutes the coat and causes [primitive markings](https://equinecolors.com/dictionary/#:~:text=a%20bay%20horse.-,Primitive%20markings,-Primitive%20markings%20may). It is believed to be the original or [wild-type](https://equinecolors.com/dictionary/#:~:text=for%20each%20gene.-,Wild%2Dtype%20allele,-The%20wild%2Dtype) allele in horses.

The **nd1-allele (non-dun1)** is [recessive](https://equinecolors.com/dictionary/#:~:text=Recessive%20Allele%20%28written%20in%20lowercase%20letters%29) to D and causes primitive markings, without the dilution.

The **nd2-allele (non-dun2)** is recessive to both and leads to an even distribution of pigment.

| **TBX3**    | **Color**                         |
| ----------- | --------------------------------- |
| **D/D**     | Chestnut dun, black dun, bay dun  |
| **D/nd1**   | Chestnut dun, black dun, bay dun  |
| **D/nd2**   | Chestnut dun, black dun, bay dun  |
| **nd1/nd1** | Chestnut nd1, black nd1, bay nd1  |
| **nd1/nd2** | Chestnut nd1, black nd1, bay nd1  |
| **nd2/nd2** | No dilution or primitive markings |

## 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_DNA1-29.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.

### TBX3

The **TBX3** gene is very important during development, controlling the **development** of the limbs, the heart, and many other tissues. It also influences **pigment** production. TBX3 does all this by, essentially, turning other **genes** **on or off**.

TBX3 tells melanocytes where **not** to put pigment in the hair. Where the gene is active, it **stops** the melanocytes from **depositing pigment** in **part** of the **hair shaft**. This leads to an **asymmetrical** distribution of pigment: one side of the hair shaft is pigmented, the other side is not. This results in a **lighter coat** color. In areas where TBX3 is not active and melanocytes are **not blocked**, the color is darker. This leads to the **primitive markings**.

Horses with **non-dun1** have a mutation that affects an **enhancer** of the TBX3 gene, which normally increases the gene's activity. As a result of the mutation, **TBX3** is **less active**, allowing pigment to be deposited evenly in the hair shaft. The coat is not visually diluted, but there is still enough TBX3 to create the **primitive markings**.

In the **non-dun2** allele, a larger section of this enhancer is deleted, and **TBX3** is almost completely **absent** as a result. Non-dun2 likely arose on a [chromosome](https://equinecolors.com/dictionary/#:~:text=Genetic%20terms-,Chromosome,-A%20chromosome%20is) that already carried non-dun1.

TBX3 affects the distribution of both **red and black** pigment.

| **Alleles** | **Mutation**                                                                                                                                                                | **Effect**                          |
| ----------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------- |
| D           | /                                                                                                                                                                           | Diluted coat and primitive markings |
| nd1         | [Point mutation](https://equinecolors.com/genetics-2-alleles/#:~:text=%F0%9F%A7%AC-,Types%20of%20mutations%3A,-Point%20mutation%3A%20One)/ substitution (g.18226905 A => G) | Primitive markings                  |
| nd2         | Deletion of 2 segments                                                                                                                                                      | No dilution/ primitives             |

## Colors

[DunDun is a dilution, located on the same gene as non-dun1\. It lightens the base coat of horses and causes primitive markings. It leads to the colors chestnut dun, black dun and bay dun.![](https://equinecolors.com/content/images/icon/favicon-111.ico)Equine ColorsHannelore Roodhooft![](https://equinecolors.com/content/images/thumbnail/dun-2-2.png)](https://equinecolors.com/dun/)

[Non-dun1Non-dun1 is located on the same gene as dun. It causes primitive markings. It leads to the colors chestnut nd1, black nd1 and bay nd1.![](https://equinecolors.com/content/images/icon/favicon-112.ico)Equine ColorsHannelore Roodhooft![](https://equinecolors.com/content/images/thumbnail/nd1-1.png)](https://equinecolors.com/non-dun1/)

**Read more:**  
[Color genes](https://equinecolors.com/color-genes/) | [MATP gene](https://equinecolors.com/matp-gene/)

[Back to Dilutions](https://equinecolors.com/dilutions/)

## Articles

1. An, T.; Dugarjaviin, M.; Han, H. ***Expression and Analysis of TBX3 Gene in the Skin from Three Locations on Dun Mongolian Bider Horse***; Genes (2024); dOI: [0.3390/genes15121589](https://doi.org/10.3390/genes15121589?urlappend=%3Futm%5Fsource%3Dresearchgate.net%26utm%5Fmedium%3Darticle&ref=equinecolors.com)
2. Cieslak J, Brooks SA, Wodas L, Mantaj W, Borowska A, Sliwowska JH, Ziarniak K, Mackowski M.; ***Genetic Background of the Polish Primitive Horse (Konik) Coat Color Variation-New Insight into Dun Dilution Phenotypic Effect***; Journal of Heredity Hered. (2021); Doi: [10.1093/jhered/esab034](https://academic.oup.com/jhered/article/112/5/436/6357767?login=false&ref=equinecolors.com)
3. Imsland, F., McGowan, K., Rubin, C. J., Henegar, C., Sundstrom, E., Berglund, J., Schwochow, D., Gustafson, U., Imsland, P., Lindblad-Toh, K., Lindgren, G., Mikko, S., Millon, L., Wade, C., Schubert, M., Orlando, L., Penedo, M. C., Barsh, G. S., & Andersson, L.; **Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses**; Nature Genetics (2015)*;* Doi: [10.1038/ng.3475](https://doi.org/10.1038/ng.3475?ref=equinecolors.com)
4. Masuda M. Tsunoda J., Nomura H., Kimura N., Altangerel G., Namkhai B., Dolj U., Yokohama Y.***; New Primitive Marking (Bider) in Mongolian Native Horse and Equus przewalskii;*** Journal of Equine Science (2007); Doi: [10.1294/jes.18.145](https://doi.org/10.1294/jes.18.145?ref=equinecolors.com)