Alleles

The TBX3 gene has three currently known alleles that influence coat color.

The dominant D-allele (dun) dilutes the coat and causes primitive markings. It is believed to be the original or wild-type allele in horses.

The nd1-allele (non-dun1) is recessive 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.

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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 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/ substitution (g.18226905 A => G)
 Primitive markings
nd2
 Deletion of 2 segments No dilution/ primitives

Colors

Dun
Dun 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.
Equine ColorsHannelore Roodhooft
Non-dun1
Non-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.
Equine ColorsHannelore Roodhooft

Read more:
Color genes | MATP gene

Back to 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
  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
  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
  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