Sunday, December 20, 2015

Primitive Markings in Dun Horses

The Primitive Markings

 © Equus ferus- Wild Horse Photography & Karen McLain Studio
Primitive markings refer to stripes and lines darker than the coat colour that appear on horses carrying the Dun trait (Dn+). The most common marking is a dorsal stripe also called a lineback. The line travels from the mane, down the back and into the tail. Many horses have a dorsal stripe but in duns, the stripe extends from the mane through the tail. There is some debate as to whether the Dun factor- the lightening of red and black on the body- is linked to a separate gene causing the primitive marking, or if they are on the same gene.
Dr. Sponenberg states that if the Dun factor and the primitive markings were located on separate genes, we would see far more horses with primitive markings who are not Duns and more Dun horses without primitive markings (Sponenberg, 2009). Neither of those situations commonly occur, so the traits are most likely located on the same gene. 

© Equus ferus- Wild Horse Photography & Karen McLain Studio
Other makings are zebra stripes on the legs, shoulder or wither stripes- some extending up the neck. Cobwebbing- or facial markings are the rarest. It is extremely rare to find a Dun without a dorsal stripe and zebra stripes are usually present but may be so pale they are not detectable except under certain circumstances. 

© Equus ferus- Wild Horse Photography & Karen McLain Studio

Guard hairs- or lighter hairs on either side of the mane may occur (see photo to the right). Horse Management Areas with a large Dun populations are Sand Wash Basin in Colorado, and Pryor Mountain in Montana, amongst others. There may be darker edges to the ears and mottling/striping on the chest or sides. The Dun factor lightens the body leaving the 'points' or lower legs, mane, and tail darker. The head is also left darker which can cause confusion when separating Duns from Roans. The Blue Roan in the photo on the right may have the Dun factor in addition to the Roan which makes identification even more challenging.

© Equus ferus- Wild Horse Photography & Karen McLain Studio
The primitive markings are found on some of the oldest horse breeds such as Sorraia, Icelandic horses, and Norwegian Fjords. They are also seen on Przewalski's Horse. However, the Dun trait is also seen on more modern breeds such as the Quarter Horse, Spanish Horse breeds and European draft breeds (Stachurska, 1999). The presence of the Dun factor does not mean the horse is from an ancient lineage- the Dun trait is autosomal dominant. This means that if the parent is homozygous (DnDn) or heterozygous (Dndn)- they will have a dun coat and pass the dun trait on to 75% off their offspring making this inherited coat colour common in isolated populations.

Sponenberg, D. (1996). Equine color genetics. Ames: Iowa State University Press.
Stachurska, A. (1999). Inheritance of primitive markings in horses. Journal of Animal Breeding and Genetics, 29-38.

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