Primitive markings, also called countershading and non-dun1, are dorsal stripes, leg barring, shoulder stripes, and cobwebbing/spiderwebbing.
There is a genetic test for primitive markings, which is called non-dun1. Research on primitive markings in horses released in 2015 discovered non-dun1. Dun (D) is the original color of horses and they have primitive markings and are diluted. Non-dun1 horses have primitive markings but are not diluted (d1). Horses without any primitive markings have a mutation called non-dun2 (d2). d1 and d2 are recessive and can appear to skip generations. d1 is dominant over d2. D is dominant and cannot appear to skip generations.
Foals often have primitive markings that is later lost after the foal coat is shed. This is presumed to be a form of camouflage.