(VERY BASIC) OVERVIEW OF HORSE COAT COLOR GENETICS Chapter 3: Dun dilution and nd1
* Make sure to zoom in! The text is quite small, so it could be difficult to read!
The dun (D), nd1 (non-dun 1), and nd2 (non-dun 2) mutations are all located in the TBX3 (T-box 3) gene. The dun dilution affects both eumelanin and phaeomelanin in phenotypically equal parts.
The dun dilution (D) is associated with diluting the fur of the main body while maintaining darker points (legs, mane/tail, often including the face), as well as creating primitive markings.
Non-dun 1 (nd1) does not typically dilute the coat, but will almost always give a horse varying degrees of primitive markings.
Non-dun 2, on the other hand, results in an entirely non-diluted coat that completely lacks all primitive markings, and so could be considered to be the "truly" non-dun allele (when homozygous).
A horse with D/- allele(s) will express the dun dilution.
A horse with nd1/nd1 or nd1/nd2 alleles will not express the (full) dun dilution, but may express primitive markings (and will have a dorsal stripe).
A horse with nd2/nd2 alleles will not express any dun dilution or primitive marking phenotype.
Bay dun is theorized to be the oldest (and most basal) coat color known in horses. This is suspected for numerous reasons; one of which being visual evidence from cave paintings (such as in the Lascaux cave) dating back to 15,000-10,000 BCE depicting what appear to be bay dun pangaré wild horses; another reason being that the last remaining truly wild horse species (the takhi, also known as the (wild) Mongolian horse or Przewalski's horse) exhibits this exact phenotype.
The nd1 (non-dun 1) allele is believed to be over 40,000 years old, whereas the nd2 (non-dun 2) allele is far more recent.
Donkeys, wild asses, and zebras (other members of the genus Equus) also express varying forms of the dun dilution gene in their wild-type coat color, even phenotypically bearing resemblance to dun horses in many cases.
Primitive markings, as mentioned, vary in placement and degrees of extremity. Each term is explained below:
↳ 'Dorsal stripes' are the most common and distinctive form of primitive marking seen in dun and nd1 horses. All dun (and non-dun 1) horses phenotypically appear to have a dorsal stripe, and may possibly have other primitive markings as well.
↳ 'Frosting' is a common phenotype in many dun horses, and refers to lighter roots (guard hairs) interspersed in the mane and tail in contrast to darker hair growing out from it. This is a trait expressed extensively in Fjord horses (who are exclusively dun dilute on any base color or cream variant).
↳ 'Leg barring’ is another common primitive marking in dun (and nd1) coats. It refers to degrees of striping on the legs, localized around the knees and hocks. Leg barring is often accompanied by ‘zippers’, which are lighter guard hairs in-between dark stripes on the legs.
↳ 'Shoulder striping' is also often seen in many duns, and appears as a varying amount of dark stripes on the withers (stemming from the dorsal stripe). This striping can get quite extensive, especially seen in nd1 Hucul/Carpathian horses.
↳ ‘Bider markings' are a relatively obscure dun-related marking, which are concentrations of darker melanin pigment around the shoulders. These markings are found in domestic dun Mongolian horses and the wild takhi (which is always bay dun). They are believed to be caused by a dominant allele within a mutation (labelled 'Bi/-').
↳ ‘Cobwebbing’ is the name for striping on the face accompanying a darker ‘mask’. Cobwebbing is not always present, but a facial ‘mask’ darker than the rest of the diluted body usually is (and is quite visible in grulla and bay dun coats). The ear and eye areas may also exhibit primitive markings related to the dun dilution.
* Some young non-dun foals show primitive markings in their foal coats that might appear similar to the markings seen in genetically dun horses. Usually, foals grow out of them as they shed into their adult coats over time. Sometimes, primitive markings in nd1 horses are seasonal.
* As stated in the image, Fjord horses have specific Norse names for their various dun coat colors: Brunblakk (bay dun), Grå (grulla/black dun or smoky grulla/smoky black dun), Rødblakk (red/chestnut dun), Ulsblakk (dunskin/buckskin dun), Gulblakk (Palomino dun), and Kvit (homozygous cream dun on any base)
Gene(s) involved: TBX3 (D, nd1, nd2)
Simplifying the possible genotypes that each of the dun phenotypes could be tested for (along with each base):
E/- A/- D/- (Bay dun) < If on seal bay base, may be called ‘brown dun’; If on Fjord horse, 'Brunblakk'
E/- a/a D/- (Grulla or black/blue dun) < If on Fjord horse, 'Grå'
E/- a/a nd1/- (Black nd1)
e/e D/- (Claybank or red/chestnut dun) < If on Fjord horse, 'Rødblakk'
BONUS cream+dun combinations:
E/- A/- D/- CR/n ('Dunskin' or buckskin dun) < If on Fjord horse, 'Ulsblakk'
E/- A/- nd1/- CR/n (Buckskin nd1 or 'nd1skin')
E/- a/a D/- CR/n ('Smoky grulla' or smoky black dun) < If on Fjord horse, 'Grå'; apparently considered the same as non-cream grulla
E/- a/a nd1/ CR/n (Smoky black nd1)
e/e D/- CR/n ('Dunalino' or palomino dun) < If on Fjord horse, 'Gulblakk'
e/e nd1/- CR/n (Palomino nd1)
E/- A/- D/- CR/CR (Perlino dun) < If on Fjord horse, 'Kvit'
E/- A/- nd1/- CR/CR (Perlino nd1)
E/- a/a D/- CR/CR (Smoky cream dun) < If on Fjord horse, 'Kvit'
E/- a/a nd1/- CR/CR (Smoky cream nd1)
e/e D/- CR/CR (Cremello dun) < If on Fjord horse, 'Kvit'
e/e nd1/- CR/CR (Cremello nd1)
Sources on dun coats and primitive markings:
↳ https://vgl.ucdavis.edu/test/dun-horse
↳https://en.wikipedia.org/wiki/Dun_gene
↳https://en.wikipedia.org/wiki/Primitive_markings
↳https://generatio.de/en/guidance/lexicon/dun-dilution-factor-horse
↳https://en.wikipedia.org/wiki/Fjord_horse
↳https://en.wikipedia.org/wiki/Lascaux
↳https://www.artchive.com/artwork/horse-cave-paintings-c-15000-10000-bc/#:~:text=The%20oldest%20painting%20in%20Lascaux,various%20rock%20shelters%20around%20Europe.
↳ (2019 article, information may be outdated!) https://www.sciencedirect.com/science/article/pii/S1751731118003506
↳ https://pdfs.semanticscholar.org/93cd/e0d5d6918502232493e19362f93145582af4.pdf
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