Non-agouti, , causes the eumelanin in the coat to extend up each hair, all over the body. This removes all ticking as well as all the lighter agouti markings. The other genes present dictate what color the pig will be.
It can be somewhat deceptive, however, as non-extension pigs (reds, creams, etc.) are shown as selfs, but are not necessarily aa. See the non-extension page for more information.