I am combining these two as the Poll Dorset was developed from the Horn Dorset. The fleeces should be very similar so I’ll have fun keeping my samples straight.
To quote the Oklahoma Livestock Breeds site http://www.ansi.okstate.edu/breeds/sheep/dorset/
“Both horned and polled Dorsets are an all white sheep of medium size having good body length and muscle conformation to produce a desirable carcass. The fleece is very white, strong, close and free from dark fiber. Dorset fleeces average five to nine pounds (2.25-4 kg) in the ewes with a yield of between 50% and 70%. The staple length ranges from 2.5 to 4 inches (6-10 cm) with a numeric count of 46’s-58’s. The fiber diameter will range from 33.0 to 27.0 microns.”
The origins of the Dorset breed is fuzzy at best. However the poll Dorset is a widely popular breed for those raising meat. It’s down-like fleece would think this breed originated in the Downs of England, and while there is a Dorset Down breed, these breeds (both the horn and polled) are separate.
Horn Dorset are quite scarce as it just plain easier to manage polled sheep. The polled gene popped up spontaneously in the gene pool of the Horned Dorsets within the United States and proliferated thanks to the help of North Carolina State College, Raleigh, NC .