It's a dominant gene, but it's a bit more than just a color. It also adds "ticking". Since it's dominant, it can be combined with other genes. When combined with Mink it makes Cinnamon, combined with Burmese makes Wheaten, etc. If homozygous, it will produce an entire litter of Agoutis when bred to an Agouti or non-Agouti. Genetic code for Agouti is A/- (AA is homozygous, Aa is heterozygous, aa is non-Agouti).
The AFRMA describes it as, "A rich chestnut with dark slate at the base of the hair. The coat is evenly ticked with black guard hairs. Belly color will be silver gray.”. Or another description from Hawthron, "The fur on an agouti’s back and flanks is multi-coloured, with bands of chestnut brown and black on a dark grey base colour with solid black guard hairs. The belly fur is ticked, but in shades of mainly grey and white which gives a silver effect overall."