Theoretically all Angus cattle should sire all polled (hornless) cattle. The American Angus Association has not allowed cattle producers to "breed up" to purebred status, like so many of the Continental breeds.
However, that being said, during the 70's and 80's when there was a lot of emphasis within the industry to increase frame size, there is speculation that some less than ethical Angus breeders did cross their Angus with taller breeds like Holstein, Simmental, etc. They then crossed these half bloods back to Angus to get more of the Angus look back into the cattle.
However, in doing this, the horn gene was introduced into the Angus breed at a very minimal level. Some commercial cattle producer friends of mine have told me that they still occasionally get horned calves out of registered Angus bulls. I know some Angus fans will deny this but it does still happen, though not very often.