Dun (D)

General Description

Dun, a dominant dilution gene, is the ancestral coat colour of equids. Both homozygous (D/D) and heterozygous (D/nd1 or D/nd2) horses display the same effects on the coat colour. The dun dilution affects both pigment types equally (unlike the cream dilution), though the points often are darker than the body. Dun horses also display “primitive markings,” including a fully pigmented dorsal stripe, leg barring, shoulder stripes, face mask, face cobwebbing, ear stripes, and/or lighter hairs along the mane and tail (“frosting”). The characteristic dilution is the result of pigments clustering asymmetrically to one side of the hair shaft. However, the lack of dun dilution is more common in most domestic horse breeds.

There are two known variants of the TBX3 gene that result in non-dun phenotypes.  Non-dun 1 (nd1/nd1 or nd1/nd2) horses lack the distinct dilution seen in dun horses, but may still display some primitive markings. Non-dun 2 (nd2/nd2) horses lack both the distinct dilution seen in dun horses as well as the primitive markings.

Genotype and Phenotype (Color Names)

Black with dun (E/_ a/a D/_): grullo or grulla

Black with non-dun 1 (E/_ a/a nd1/_): black with primitive markings

Black with non-dun 2 (E/_ a/a nd2/nd2): black

Bay with dun (E/_ a/a D/_): bay dun

Bay with non-dun 1 (E/_ a/a nd1/_): bay with primitive markings

Bay with non-dun 2 (E/_ a/a nd2/nd2): bay

Chestnut with dun (e/e a/a D/_): red dun

Chestnut with non-dun 1 (e/e a/a nd1/_): chestnut with primitive markings

Chestnut with non-dun 2 (e/e a/a nd2/nd2): chestnut

* Other dilutions, modifiers, and spotting genes can further alter the colour of a horse – for simplicity, we have not named all possible combinations!

Gene Information

TBX3 is a transcription factor that is critical for normal development. Mutations in other species cause abnormal limbs, genitals, teeth, and apocrine glands. The nd1 mutation (a single base change) and the nd2 mutation (a large deletion) alter the regulation of TBX3 in keratinocytes during hair growth. The negative phenotypes observed in other species have never been noted in non-dun horses.

References

Imsland F et al., “Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage colour in horses.” (2016) Nat Genet. 48: 152-8.

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