What is a dorsal stripe in horse marking?

A dorsal stripe is indeed characterized as a dark or black line that runs along the horse's back, specifically from the withers to the croup. This marking is a distinctive characteristic often seen in certain horse breeds and can indicate a horse's genetic background. It is typically more pronounced in certain coat colors, especially in primitive breeds or those with a "dun" coloration. The presence of a dorsal stripe can serve not only as a unique visual trait but also as a marker of a horse's lineage or genotype.

The other options describe different types of markings or features that are not associated with the dorsal stripe, such as face markings or patterns on the coat, which do not relate to the definition or characteristics of a dorsal stripe.