This gene can work on both red and black base colors by lightening the base color. However, it does not lighten the base color of the mane, tail, legs and the primitive markings, including the dorsal stripe, the zebra stripe and the shoulder stripe often seen in donkeys.
A Dun horse always has a dorsal stripe with or without the addition of a cream dilution. Combinations of a cream diluted bay (buckskin) and a dun diluted bay (bay dun) produce a Buckskin Dun. The dorsal stripe shows that the horse is a dun, but the genetics will show a cream dilution with a lighter hair coat color, especially on the face.
Chestnut with the dun gene will have a darkened hair coat along the back (the dorsal stripe) and darkened ears, mane, tail and lower limbs. These markings are of their base coat and look dark relative to the lightened remaining coat color.
In essence, the dun gene lightens the coat color except for the dorsal stripe, the ears, mane, tail, legs and any ancient markings such as the shoulder stripe of donkeys and the zebra leg markings of horses and donkeys.
The grullo (aka grulla) is a dun dilution of the black horse. Each hair retains the base coat but adds a lighter color creating a tan-gray coloration. Grullo hair (two colors on one hair) is different from gray hair (all depigmented over time), and roan hair (either pigmented or depigmented with no change in this over time). Grullos are also called blue duns, gray duns or mouse duns.
There is a caveat concerning the dorsal stripe. All duns must have a dorsal stripe, but not all horses with a dorsal stripe are duns because the primitive markings (dorsal stripe, zebra markings, leg stripes, shoulder line) can also be expressed more in some horses that carry one of the two non-dun genes. I know this isn’t very clear, but that’s genetics!
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